How Much Does It Cost to Mail a 9x12 Envelope? (Price Guide)

A 9x12 envelope is bigger than a standard letter, so USPS prices it as a large envelope, also called a flat. The postage is higher than a regular stamp, and it climbs with each ounce.

The prices below are from USPS Notice 123, the official price list effective October 4, 2026, and were checked on Oct 14, 2026. USPS changes prices about twice a year, so see the current Notice 123 for the latest numbers.

First-Class Mail flats are priced by the ounce up to 13 ounces. A few sheets of paper usually fit in the first bracket, while a thick packet of documents moves up.

Weigh the sealed envelope on a kitchen or postal scale, since the envelope and its contents both count. Stay below the ounce line when you can, as just over it moves you into the next price.

A letter has to fit in size limits and be flexible enough to run through sorting machines. A 9x12 envelope is larger, so it needs different handling and a higher price. Square corners, even thickness and flexibility also matter. A flat that is stiff, lumpy or uneven may be treated as a package, which costs more. If your envelope holds a rigid item, such as a ring binder or a photo with cardboard, ask the post office to measure it.

Once the envelope passes 13 ounces, First-Class Mail no longer applies. Heavier envelopes ship under a different service. Priority Mail Flat Rate envelopes are priced by the container, not by weight, and Ground Advantage is priced by weight and zone.

A flat rate envelope is a good fit for a dense, heavy stack of paper, since the price stays the same however much it holds within the limits. Ground Advantage may cost less for a lighter parcel going a short distance.

• Weight. Every additional ounce adds to the price up to the 13 ounce limit for First-Class flats.
• Size and shape. Envelopes that are too stiff or uneven may be priced as packages.
• Service. First-Class is cheapest for light envelopes, while Priority Mail Flat Rate suits dense ones.
• Extras. Tracking, certified mail and insurance cost extra, and a return receipt is another fee.
• Where you buy postage. Stamps bought at the counter work, and printed postage from an online service uses the same retail price table.

The simplest approach is to take the sealed envelope to a post office and have the clerk weigh it, which avoids the risk of too little postage. If you mail often, a postal scale at home pays for itself in avoided trips. Stamps work for flats, and printed postage from an online service can also be used. Short postage is the main reason envelopes come back, so when the weight is close to a line, round up.

Use a rigid backer such as a piece of cardboard if the contents should not bend, because damage from bending is a common problem. Write "Do not bend" clearly if needed. Put the address in the center and the return address in the upper left corner. For anything important, pay for tracking so you can confirm that it arrived.