Common USPS postage mistakes (and how to avoid them)
Published April 17, 2026
Quick answer: fix these first
- Weigh sealed: the mailed piece, not a loose card.
- Pick the right product: letter vs flat vs postcard—letter vs flat vs parcel.
- Check surcharges: square and rigid mail can add letter surcharges; flats can fail if stiff—see non-machinable mail and flat envelope rules.
Mistake 1: One Forever stamp “because it usually works”
A Forever stamp covers the first ounce of a qualifying letter—not every mailpiece. At two ounces, a machinable letter typically needs more than one stamp's worth of postage on the ladder used here. Use 2 oz letter postage or the home calculator.
Mistake 2: Treating a 9×12 like a letter
A 12″ edge often exceeds letter length limits in our model. If your mail is a flat, use flat retail—start with 9×12 envelope postage explained.
Mistake 3: Ignoring square envelopes
Square letters often trigger non-machinable handling. Read square envelope postage and add the surcharge in the calculator when it applies.
Mistake 4: Forgetting RSVP and reply mail
Events mail often needs two postage decisions: outer envelope and reply card. The wedding invitation postage guide walks through typical stacks.
Related guides on this site
Sources & methodology
Dollar amounts for First-Class retail on this site are drawn from the same USPS Notice 123 tables used by the calculators (domestic single-piece retail letter, large envelope (flat), and standard postcard rows where applicable). The tools classify mail from your measurements and flags—they do not price Priority Mail, Media Mail, or zone-based parcels. For anything unusual, confirm at a USPS retail location or with official USPS products.