Barcode Label Printing for Shipping Compliance
Why Shipping Labels Get Rejected
Every major retailer and carrier has specific barcode label requirements, and they enforce them. Amazon charges vendors when labels fail to scan at their fulfillment centers. Walmart issues chargebacks for non-compliant GS1-128 shipping labels. FedEx and UPS reject packages with unreadable barcodes, delaying shipments and creating rework. If your warehouse ships to any of these partners, your thermal printer output has to meet their standards or you pay for it.
Most compliance failures come down to the same handful of problems: wrong barcode symbology, poor print quality, incorrect label placement, or missing data fields. MIDCOM Data Technologies works with shipping operations across the U.S. and Canada that handle hundreds to thousands of packages per day. Here is what goes wrong most often and how to fix it.
Barcode Symbology: Using the Right Format
Different partners require different barcode types, and using the wrong one is the fastest way to get a shipment flagged.
GS1-128 (formerly UCC/EAN-128)
GS1-128 is the standard barcode symbology for shipping labels in retail supply chains. Walmart, Target, Costco, and most major retailers require it on every carton label. GS1-128 encodes application identifiers (AIs) that carry structured data: SSCC-18 serial shipping container codes, purchase order numbers, ship-to postal codes, and item quantities. If your label software is printing Code 128 without the proper AI structure, the barcode will scan but the data will not parse correctly in the retailer’s system — and that still counts as non-compliant.
Code 128 and Code 39
FedEx and UPS use their own proprietary label formats, but when they require you to include a reference barcode (like a PO number or customer SKU), Code 128 is usually the expected symbology. Code 39 is older and less data-dense but still shows up in some legacy systems. If you are generating carrier labels through an API or shipping software (ShipStation, EasyPost, etc.), the symbology is handled automatically. Problems arise when warehouses print supplemental labels on their own thermal printers with incorrect barcode settings.
2D Codes: QR and Data Matrix
Amazon’s FNSKU labels use Code 128, but their transparency and routing labels increasingly include 2D barcodes. GS1 has announced a 2027 sunrise date for wider adoption of GS1 Digital Link QR codes at point of sale, which will eventually affect shipping workflows too. If your scanners only read 1D barcodes, that is a problem you should start planning for now. MIDCOM’s barcode scanner catalog includes 2D-capable models from Zebra and Honeywell that handle both legacy 1D and newer 2D codes.
Print Quality: The Difference Between Scanning and Not Scanning
A barcode can look fine to the human eye and still fail a scanner. Print quality is measured by the ANSI/ISO grading system on a scale from A (best) to F (fail). Most retail partners require a minimum grade of C, and some specify B. FedEx requires that barcodes scan on the first pass at their sort facilities — there is no grade, just pass or fail at belt speed.
What Kills Print Quality
The usual culprits:
Darkness set too high or too low. Too much heat smears the bars and closes the quiet zones. Too little heat produces faint bars that lack contrast. The correct darkness setting is the lowest value that produces a solid, high-contrast print. Test with a verifier, not your eyes.
Worn printheads. A printhead with dead elements produces white voids (thin vertical lines) through the barcode. These voids can make individual bars unreadable. If cleaning the printhead does not eliminate the voids, the printhead needs replacing. Printheads are consumables — on a Zebra ZT411 printing 2,000 labels per day, expect to replace the printhead every 12 to 18 months.
Wrong media and ribbon combination. Using a wax ribbon on a synthetic label (or a resin ribbon on a paper label) produces poor adhesion and inconsistent density. Match your ribbon type to your label material. MIDCOM’s consumables team can recommend the right combination for your specific application.
Print speed too fast for the resolution. A 203 dpi printer running at maximum speed may not apply enough energy per dot to produce clean bar edges. Slowing the print speed by one or two increments often fixes borderline quality issues without measurably affecting throughput.
Label Size, Layout, and Placement
A perfect barcode on the wrong size label or in the wrong spot on the carton still gets rejected.
Amazon Requirements
Amazon FBA requires FNSKU labels on individual items (1″ x 2″ or 1″ x 3″ depending on the product) and shipping labels on cartons (4″ x 6″ standard). Carton labels must be placed on a flat surface, not wrapping over an edge or seam. The barcode must have at least a 1/8″ quiet zone (blank space) on each side. Amazon also requires that any existing barcodes on the carton be covered or removed to prevent mis-scans at the fulfillment center.
Walmart Requirements
Walmart’s shipping label requirements follow the GS1 standard for logistics labels. The label must be 4″ x 6″ and placed on the largest flat surface of the carton, at least 1.25″ from any edge. The GS1-128 barcode must encode the SSCC-18, and the human-readable interpretation must be printed below the barcode. Walmart’s compliance team audits inbound shipments and issues chargebacks for labels that do not meet these specifications.
Carrier Requirements
FedEx and UPS both use 4″ x 6″ thermal labels as their standard. The label must be applied flat on the largest side of the package with no wrinkles, bubbles, or damage to the barcode area. Clear shipping tape should not be applied directly over the barcode — the reflection from glossy tape can cause scan failures at high-speed sort facilities. If you are taping over labels for moisture protection, use matte tape or position the tape above and below the barcode, not over it.
Verifying Before You Ship
The cheapest compliance failure is the one you catch in your own facility. A barcode verifier (not just a scanner) grades the barcode against ISO/ANSI standards and tells you exactly what is wrong — low contrast, narrow bars too thin, insufficient quiet zone, etc. A scanner just tells you pass or fail.
For operations shipping 500+ packages per day, a verifier at the end of the packing line catches problems before the carrier picks up. For smaller operations, spot-checking 1 in 20 labels with a verifier is often enough to identify systematic issues like a failing printhead or incorrect darkness settings.
If verification is not in your budget, at minimum scan every label with the same scanner model your receiving partner uses. Amazon’s fulfillment centers use Zebra and Honeywell fixed-mount scanners — if your barcode scans reliably on a comparable unit, it will likely pass at their facility.
Common Compliance Mistakes and What They Cost
Amazon’s “Prep and Label” chargebacks run $0.55 to $1.00+ per unit when they have to relabel your inventory. At 1,000 units per shipment, that is $550 to $1,000 per incident. Walmart’s chargeback schedule is similar, with fees for non-compliant labels, missing ASN data, and late shipments caused by labeling errors.
The less obvious cost is time. When a carrier rejects a package for an unreadable label, someone on your team has to reprint, relabel, and rebook the shipment. Multiply that by 20 rejected packages in a day and you have lost a significant chunk of labor productivity.
Most of these costs go away once your printer settings are dialed in and your label formats are verified against each partner’s spec. MIDCOM’s on-site service technicians can audit your print setup, verify your label formats against partner requirements, and configure your printers for consistent compliant output.
Frequently Asked Questions
What barcode type do I need for Amazon FBA shipping labels?
Amazon FBA uses Code 128 for FNSKU item labels and their standard carrier barcode format for shipping labels. If you are generating shipping labels through Amazon’s Seller Central or a shipping API, the format is handled automatically. For FNSKU labels printed on your own thermal printer, configure your label software for Code 128 symbology at a minimum X-dimension of 10 mil (0.010 inches) and include proper quiet zones of at least 1/8 inch on each side.
Why do my shipping labels keep getting rejected by Walmart?
The most common reasons are: missing or incorrectly formatted SSCC-18 data in the GS1-128 barcode, label placed too close to a carton edge (must be at least 1.25 inches from any edge), insufficient barcode print quality (minimum ANSI grade C), and mismatches between the ASN data transmitted electronically and the data encoded in the barcode on the physical label. A barcode verifier can identify print quality issues, while an EDI audit catches data mismatches.
How do I check barcode print quality on my thermal printer?
Use a barcode verifier, which grades the barcode against ISO/IEC 15416 (1D) or ISO/IEC 15415 (2D) standards. A verifier measures contrast, modulation, edge determination, and quiet zones — not just whether the code scans. Webscan, REA, and Axicon all make verifiers suitable for warehouse use. If a full verifier is not in the budget, at minimum use a scanner identical to what your shipping partner uses and test from the same distance and angle their equipment operates at.
Does label material affect shipping compliance?
Yes. Direct thermal labels are the standard for shipping because they do not require ribbon and produce good contrast at typical warehouse print speeds. However, direct thermal labels are sensitive to heat and friction — if packages sit on a hot loading dock or go through a rough sort facility, the barcode can fade or smear. For shipments that may be exposed to extreme conditions, thermal transfer labels with a wax-resin ribbon offer better durability. The tradeoff is slightly higher cost per label and the need to manage ribbon inventory.
Can my existing thermal printer handle compliance requirements?
Most industrial thermal printers from Zebra, Honeywell, and SATO can produce compliant shipping labels if they are properly configured and maintained. The key factors are print resolution (203 dpi is sufficient for standard shipping barcodes, 300 dpi for smaller or denser codes), correct darkness and speed settings, and a printhead in good condition. If your printer is more than 5 years old and struggling with consistent print quality, a professional service visit can determine whether a tune-up or a replacement makes more sense.
Get Your Labels Right the First Time
Compliance chargebacks and rejected shipments are preventable once you nail down printer settings, verify your label formats, and spot-check output before it ships. For a print audit, label format review, or help configuring your printers for specific partner requirements, contact MIDCOM Data Technologies at 866-696-3458. We work with shipping operations across the country to make sure what goes out the door scans at the other end.