Prep, labeling, and barcode compliance are not exciting topics. They are, however, the topics that determine whether your inventory makes it through receiving, gets stowed correctly, and reaches customers without generating returns, chargebacks, or account health issues. Getting this wrong costs real money and real time, and the mistakes are almost always avoidable.
This guide covers the specific requirements that matter for Amazon FBA and Walmart WFS, the common mistakes that create problems, and the operational standards you need to meet to keep your supply chain clean.
Barcode Fundamentals: UPC, EAN, GTIN, and FNSKU
Before anything gets prepped or labeled, you need to understand the barcode landscape, because using the wrong barcode in the wrong context is one of the most common sources of inventory confusion.
UPC (Universal Product Code). This is the 12-digit barcode issued by GS1 that identifies a product universally. Every distinct product configuration (size, color, flavor, count) needs its own UPC. UPCs must be purchased from GS1 directly. Third-party UPC resellers (the ones selling barcodes for a few dollars each) issue codes from GS1 prefixes they do not own. These work until they do not, and when they fail, it is usually at the worst possible time: a listing gets merged with another product, or a barcode validation check suppresses your listing.
EAN (European Article Number). The 13-digit equivalent of the UPC, common for products manufactured outside the US. Amazon accepts EANs as product identifiers, but you should verify that the EAN is properly registered in GS1 and linked to your brand.
GTIN (Global Trade Item Number). The umbrella term that includes UPC, EAN, and other barcode formats. When Amazon or Walmart asks for a GTIN, they are asking for whatever standard barcode format applies to your product.
FNSKU (Fulfillment Network Stock Keeping Unit). This is Amazon's internal barcode that ties a specific unit to a specific seller's inventory. FNSKUs are generated when you create a listing on your Amazon seller account and elect to use Amazon's labeling. The FNSKU is unique to your account, which means a unit labeled with your FNSKU can only be attributed to your inventory, not commingled with other sellers' stock.
FNSKU vs Manufacturer Barcode: The Commingling Decision
Amazon gives sellers a choice: use the manufacturer barcode (UPC/EAN) for FBA inventory, or use FNSKU labels. This choice has significant implications.
Manufacturer Barcode (Commingled Inventory). If you elect to use the manufacturer barcode, your inventory is commingled with other sellers' units of the same product. When a customer orders from you, Amazon may fulfill it with a unit that was sent in by a different seller. This creates risk. If another seller sent in counterfeit, damaged, or expired product, your customer may receive it, and the resulting negative review or A-to-Z claim hits your account.
FNSKU (Stickerless Commingling Disabled). If you use FNSKU labels, every unit you send to FBA is tracked as yours. Customers who buy from you receive units from your inventory. This eliminates the commingling risk. The trade-off is that every unit needs an FNSKU label applied, which adds a step to your prep process.
Our recommendation: Always use FNSKU labeling. The incremental cost of labeling is trivial compared to the risk of commingling issues. One counterfeit complaint or one expired unit sent to a customer from commingled inventory can trigger an ASIN-level investigation that takes your listing offline for weeks.
Labeling Requirements and Standards
Whether you are applying FNSKU labels, suffocation warning labels, or set labels, the standards are specific and enforcement is real.
FNSKU Label Specifications.
- Size: 1 inch by 2 inches minimum, or 1 inch by 3 inches for readability
- Print quality: Must be scannable at first pass. Thermal labels printed on a direct thermal printer (like a DYMO or Rollo) work well. Inkjet-printed labels on standard paper often fail scan tests
- Placement: On a flat, scannable surface. Not on a curve, not on a seam, not on a shrink-wrapped edge that distorts the barcode
- Coverage: The FNSKU label must be the only scannable barcode on the unit. If the manufacturer barcode is visible, it must be covered, either by the FNSKU label itself (if placed directly over it) or by an opaque sticker
- Content: The FNSKU barcode, the product title (truncated is fine), and the condition (New)
Expiration Date Labels. For products with expiration dates (food, supplements, beauty), the expiration date must be visible on the outside of the unit in MM/DD/YYYY or MM/YYYY format. Amazon requires at least 90 days of remaining shelf life at the time of receiving for most categories (105 days for some). If the expiration date is not clearly printed on the product packaging, you need to apply a separate expiration date label.
"Sold as Set" Labels. If your product is a multi-pack or bundle that should not be separated, it needs a "Sold as Set" or "Ready to Ship" label. This prevents fulfillment center workers from breaking the set apart and stowing the components individually.
Country of Origin Labels. US Customs requires country of origin marking on imported products. If the product packaging does not include this, you need to apply a label. This is a customs and regulatory requirement, not just a marketplace one.
Poly Bagging Requirements
Poly bagging is required for specific product types to protect the product and to prevent it from creating problems in the fulfillment center.
When Poly Bagging Is Required:
- Products with loose parts that could separate during storage or shipping
- Fabric or textile products (apparel, bedding, plush items)
- Products in packaging that is not sealed or could open during handling
- Liquids in containers that could leak (even if unlikely)
- Products that could be damaged by dust, moisture, or contact with other items
Poly Bag Specifications:
- Minimum 1.5 mil thickness (this is the most commonly violated spec; bags thinner than 1.5 mil are rejected)
- Clear or transparent
- Must be sealed (heat sealed, adhesive closure, or resealable zip closure)
- Must have a suffocation warning if the bag opening is 5 inches or larger
Suffocation Warning Requirements. This is a consumer safety regulation, not just a marketplace rule. The warning text must be printed on the bag or applied as a label, and it must be legible. The standard text is:
"WARNING: To avoid danger of suffocation, keep this plastic bag away from babies and children. Do not use this bag in cribs, beds, carriages, or playpens. This bag is not a toy."
The text size requirements vary by bag dimensions. For bags under 2 inches by 2 inches, the warning must be at least 6-point type. For larger bags, it scales up. The specific requirements are defined by the states that enforce suffocation warning laws, with New York and California being the most commonly referenced.
Common Poly Bagging Mistakes:
- Using bags that are too thin (under 1.5 mil). This is the most common chargeback trigger for poly bagging.
- Forgetting suffocation warnings on bags with openings 5 inches or larger
- Leaving excess air in the bag so the product does not lay flat (creates stowing problems)
- Using bags that are too large for the product, which wastes space and creates receiving issues
Case Pack Rules and Inbound Shipping
How you pack and ship inventory to fulfillment centers is subject to specific rules that, when violated, create receiving delays and chargebacks.
Single-ASIN Case Packs. A case pack should contain only one ASIN. Mixed-ASIN cases are allowed but are handled differently and typically route through Amazon's less efficient receiving process, which means longer check-in times. If you are shipping multiple ASINs, pack each ASIN in its own case.
Case Pack Quantity Declaration. When you create a shipping plan, you declare how many units are in each case. The actual case content must match this declaration exactly. If you declare 12 units per case and one case has 11, the discrepancy will be flagged during receiving. Depending on the fulfillment center and the time of year, this can result in a manual count that delays the entire shipment.
Case Dimensions and Weight. Amazon limits individual case weight to 50 pounds. Cases over 50 pounds require a "Team Lift" label. Cases over 100 pounds require a "Mechanical Lift" label and must be palletized. Case dimensions should not exceed 25 inches on any side for standard processing. Oversized cases may require special handling.
Box Content Information. Amazon gives you the option of providing box content information when creating a shipping plan. Providing accurate box content information speeds up receiving because Amazon can scan one box label and know exactly what is inside, rather than opening and scanning each unit. This option saves days on receiving time and should always be used.
Pallet Requirements (for LTL and FTL Shipments). If you are shipping full pallets, they must be standard GMA pallets (40 by 48 inches), stackable, stretch-wrapped on all four sides with clear wrap, and labeled with Amazon pallet labels on all four sides. Pallet height including the pallet itself should not exceed 72 inches for stacking purposes, though 50 inches is preferred for floor-loaded containers.
Walmart WFS-Specific Requirements
Walmart Fulfillment Services has its own prep and labeling standards that overlap with Amazon FBA in some areas and diverge in others.
Item Labels. WFS uses its own item labels, similar to FNSKU but generated through Walmart's system. Each unit needs a WFS item label applied per Walmart's specifications. The label must be scannable and placed on a flat surface.
Packaging Requirements. WFS requires that all products arrive in "retail-ready" condition. This means the product should be in its final retail packaging, not in generic brown boxes or loose packaging. If the product does not have retail-ready packaging, it needs to be packed in a way that meets Walmart's presentation standards.
Case Pack Labels. WFS requires specific case-level labels that include a barcode, the item name, the quantity, and the lot number if applicable. These labels must be generated through Walmart's Seller Center system.
Shipping Requirements. WFS shipments must be sent to the designated receiving center. Unlike Amazon, WFS currently has a more limited fulfillment center network, so shipments are typically routed to a single location per shipping plan.
Chargebacks: What Triggers Them and What They Cost
Amazon charges prep-related fees when products arrive at the fulfillment center and do not meet requirements. These are not arbitrary fees. They represent the cost of Amazon having to fix your mistakes.
Common Chargeback Triggers:
- Missing FNSKU labels: Amazon applies a label and charges a per-unit fee
- Missing poly bagging: Amazon bags the unit and charges a per-unit fee
- Missing suffocation warning: Product may be rejected or Amazon applies warning and charges a fee
- Barcode not scannable: Manual processing fee
- Wrong case pack quantity: Manual count fee
- Overweight cases without proper labeling: Handling surcharge
- Expiration date not visible: Product may be rejected or held for manual inspection
Typical Chargeback Costs. Individual chargeback fees range from 0.20 to 1.50 dollars per unit depending on the type of non-compliance. This may sound small, but across a shipment of 500 units, a per-unit chargeback of 1.00 dollar adds 500 dollars in unplanned cost. Across a year of shipments, these fees can represent a meaningful margin erosion. More importantly, repeated non-compliance can result in your account losing the ability to self-prep, which means Amazon preps everything for you at their rates, which are significantly higher than doing it correctly yourself.
Prevention Is the Only Economical Strategy. The cost of compliant prep, including labels, poly bags, suffocation warnings, and quality checks, is a fraction of the cost of chargebacks and receiving delays. A thermal label costs less than a penny. A poly bag with suffocation warning costs 5 to 15 cents. The chargeback for not using them costs 10 to 100 times more.
If you are unsure whether your current prep process meets marketplace requirements, we can review it. See our full operational capabilities or request a Snapshot assessment for a detailed review of your inbound process.
Building a Prep Standard Operating Procedure
Every product in your catalog should have a documented prep SOP that specifies:
- Barcode type and placement (FNSKU, location on unit, covering requirement for manufacturer barcode)
- Poly bagging requirement (yes or no, bag size, mil thickness, suffocation warning requirement)
- Additional labeling (expiration date, sold as set, country of origin, Prop 65, fragile)
- Case pack configuration (units per case, case dimensions, case weight, case label requirements)
- Special handling (bubble wrap, fragile stickers, orientation labels, do not stack)
This SOP should be created once per ASIN and reviewed whenever the product packaging changes or marketplace requirements update. The time invested in building these SOPs saves exponentially more time in avoiding chargebacks, rejected shipments, and customer complaints.
At Oxalis, prep SOPs are part of our standard onboarding for every product we handle. If you are a brand or distribution partner looking for an operator that takes this seriously, apply to work with us or contact our team directly.
FAQ
What is the difference between FNSKU and UPC, and which should I use for FBA?
UPC is a universal product identifier issued by GS1 that identifies what the product is. FNSKU is an Amazon-specific barcode that identifies whose inventory a specific unit belongs to. For FBA, you should use FNSKU labeling on every unit. This prevents your inventory from being commingled with other sellers' stock, which protects you from counterfeit claims, expiration issues, and quality problems caused by other sellers' inventory.
How much does it cost to fix prep issues after inventory is already at the fulfillment center?
If Amazon catches a prep issue at receiving, they charge per-unit fees ranging from 0.20 to 1.50 dollars to fix it. If the issue is not caught at receiving and results in a customer complaint or return, the cost is much higher: the return processing fee, the potential refund, the negative review impact, and the possible ASIN-level investigation. If you need to remove inventory from the fulfillment center to re-prep it, you pay a removal order fee of 0.50 to 1.00 dollar per unit plus the shipping cost to get it back, re-prep it, and ship it back to FBA. This can easily cost 3 to 5 dollars per unit total.
Do Walmart WFS prep requirements differ significantly from Amazon FBA?
The fundamentals are similar, but the specifics diverge. WFS uses its own item label system rather than FNSKU. WFS has stricter retail-readiness packaging requirements. Case labeling format and content differ. Shipping plan creation and routing work differently. If you sell on both platforms, you need separate prep SOPs for each, and ideally you prep Amazon and Walmart inventory in separate batches to avoid cross-labeling errors.
What is the most common prep mistake that leads to chargebacks?
Missing or improperly applied FNSKU labels are the single most common chargeback trigger, followed closely by poly bagging that does not meet the 1.5 mil thickness requirement. Both of these are simple to prevent with a basic quality check before shipment. The most expensive mistake, though less common, is declaring the wrong case pack quantity, because it delays receiving for the entire shipment rather than just the non-compliant units.