Additional-ounce stamps and USPS letters (what flats do differently)
Published April 17, 2026
Quick answer: how additional ounces fit letters
- Letters: after the first ounce, each additional billable ounce adds $0.29 on the retail ladder used here—often paired with a Forever-class stamp for the first ounce.
- Flats: use the large envelope total for your billable ounce tier—not the letter first ounce + letter additional-ounce pattern.
- Forever stamps: apply at their current face value toward whatever postage is due; “how many stamps” is a division problem against the required total, not a promise that one Forever always covers one ounce of every product.
Why people confuse additional-ounce stamps with flats
Both letters and flats are priced in ounces, but the dollar schedule is different. A flat's 2 oz total is not automatically “first ounce letter + one additional ounce” in dollars. Use large envelope postage when you are in flat class.
Worked letter examples (calculator pages)
- 2 oz letter postage
- 3 oz letter postage
- Home calculator for your exact flags
FAQ
Can I put additional-ounce stamps on a 9×12 flat?
You can affix any valid USPS postage, but you must still meet or exceed the flat retail total. Letter additional-ounce stamps are not a short-cut label for “flat add-on postage”—see how many stamps for a 9×12.
Does rounding work the same for letters and flats?
This site uses whole billable ounces on the published ladders—fractional weight rounds up. The important part is which table applies, not the rounding rule alone.
Sources & methodology
Dollar amounts for First-Class retail on this site are drawn from the same USPS Notice 123 tables used by the calculators (letter and large envelope (flat) retail ladders used in calculator outputs). 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.