USPS letter vs flat vs parcel (what this site covers)
Published April 17, 2026
Quick answer: three buckets
- Letter: within size/weight limits (this site uses up to 3.5 oz and length up to 11.5″ among other caps)—priced on the letter ounce ladder plus applicable surcharges.
- Large envelope (flat): exceeds letter limits but can still qualify for First-Class flat retail when flexible and uniformly thick—priced on the flat table through 13 oz in our model.
- Parcel / not modeled here: rigid, uneven, or oversized pieces may need Priority Mail or other services—our tools warn instead of inventing a zone price.
Why the distinction matters for postage
Letter and flat retail are different products on Notice 123. Mixing them up—especially putting letter “additional ounce” stamps on a flat—is one of the fastest ways to underpay. If you are unsure, start on the home calculator with Mail class set to Auto so letter vs flat follows your measurements.
Where to go next on this site
- Letter: 1 oz and 2 oz guides
- Large envelope (flat) hub
- Non-machinable surcharge (letters—not the same as flat rigidity rules)
- Postcard size and pricing
USPS references
Official shape and eligibility language lives in the USPS Domestic Mail Manual and Notice 123 price tables. Use those sources when a clerk disagrees with an estimate.
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.