If you're sending square-shaped stationery or a specialty envelope, you should put an extra stamp on it.

All postcards and envelopes must be rectangular. Shipping costs vary depending on weight, or you can purchase a priority mail box, which ships at a flat rate if it fits inside the USPS-produced boxes. Usually, the first four ounces need a minimum of $3.80 in postage, with more postage as weight increases. The number of required stamps depends on the weight and size of the package. If your letter is one ounce or below, you will need one Global Forever stamp or three Forever Stamps. For mail going outside of the United States, you can use Forever Stamps or a Global Forever Stamp. This includes 9x12, legal, and manila envelopes, Two stamps are needed for the first ounce, and postage will be an additional $0.20 cents after. If your letter is a standard-sized envelope and weighs 3.5 ounces or less, you will need one stamp. If your envelope isn't flat, it won't fit in automated processing equipment, and you will be charged more. When figuring out how many stamps you need to mail your letter or postcard, your postage costs will depend on the shape and size of your mail.

That changes for irregularly shaped envelopes, heavy envelopes, or oversized envelopes. In most cases, a single postage stamp will suffice to mail a letter. There's no easy answer to how many stamps your mail item needs because it's dependent on many different things such as weight, size, and the receiver's location. If you don't have the time to go to a post office before calculating how many stamps you need, here's a guide that will help you. The number of stamps you put on letters or packages depends on the envelope size and its weight.

Dimensions usps small flat rate box