A low scan rate feels like a mystery — the barcode looks fine to you, the order shows up correctly in your dashboard, but USPS never seems to read it. Before assuming the worst, it's worth knowing that a valid barcode can still go unscanned for reasons that have nothing to do with the barcode's data — they're almost always about the physical piece it's printed on.
The most common cause: how the label is attached to the envelope
If you're printing on plain paper and attaching it to the envelope yourself — rather than using a windowed envelope, where the barcode shows through a fixed opening — how you attach it matters more than most sellers expect.
Tape can cause glare that blocks the scan
USPS reads barcodes optically, at high speed, as mail moves through automated sorting equipment. Clear or glossy tape placed directly over — or even near — the barcode can create glare that interferes with that optical read, even when the print underneath is perfectly crisp and legible to your own eye. The barcode can look completely fine and still fail to scan.
A taped-on slip isn't flat, and that has its own cost
Beyond glare, tape creates a slightly raised, uneven surface. Pieces that aren't flat enough can get kicked out of automated processing and routed to manual handling instead — which usually means fewer scan touchpoints along the way, not zero delivery, but a much sparser tracking record. A piece that gets handled manually for part of its journey will often show gaps in scan history that look alarming but don't reflect an actual problem with delivery.
Print alignment and skew
USPS's scanning equipment has a real, limited tolerance for how rotated a barcode can be before it becomes unreadable. Even a few degrees of tilt can be enough to drop a piece out of an automated read.
This is more likely to happen with manual, one-at-a-time processes than with a consistent batch workflow — hand-feeding an envelope, or hand-aligning a cut slip, introduces more positional variance than a fixed, repeatable process does. A windowed envelope sidesteps this almost entirely, since the printed sheet folds into the same position relative to the same window opening every time.
Print quality and contrast
Low ink levels, faint toner, or printing onto anything other than plain white paper can all reduce the contrast a scanner needs to read the barcode reliably. This one's easy to overlook because a barcode that looks readable on screen or under normal light doesn't always scan the way it prints.
A quick way to check what's going on
| What you see | Likely cause |
|---|---|
| Consistently low scan rate across most orders | Alignment, tape glare, or print contrast — something systemic in your process |
| Scan rate fine, but a few specific pieces never scan | Often normal facility-level coverage gaps — not necessarily something to fix |
| Barcode looks visibly crooked in a photo | Print/feed alignment — check how the piece is being fed or positioned |
| Tape directly over or near the barcode | Glare — try adhesive instead, or reposition tape away from the code entirely |
If you want the most complete fix
Every cause above comes from something happening between your printer and the final mailpiece — feeding, attaching, aligning. A windowed envelope removes that step entirely: the barcode is printed on a flat sheet exactly the way your printer is designed to print, and the envelope itself never goes near the print process at all. If switching formats is workable for you, it's the change least likely to leave anything to chance.
If it isn't — adhesive shipping labels are the next best thing. They solve the flatness and glare problems tape introduces, and they're generally easier to place accurately than a hand-cut slip lined up by eye.
Quick answers
Does a low scan rate mean my mail wasn't delivered?
No. Scans and delivery aren't the same thing — plenty of mail is delivered successfully without ever getting an intermediate scan. A missing scan is a visibility gap, not evidence of non-delivery.
Should I switch to windowed envelopes?
If it's workable for your process, it removes the most common causes above in one move. If it isn't, focus on adhesive over tape and a consistent, repeatable print process first.
Is batch printing actually more reliable than printing one at a time?
It's a reasonable theory — a continuous run through a feeder can settle into more consistent alignment than single, disconnected feeds — but it depends on your specific printer and setup. If you have a feeder tray, it's worth comparing the first piece in a run to the later ones yourself.