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These are the maximum length and height measurements for large envelopes. Anything larger than this qualifies as a parcel, and it'll cost more to send. Place the item you're sending into your envelope. Once it's in there, use a ruler to see how thick the envelope is. If it's too thick, the post office will define it as a parcel. Try to make sure the item is the same amount of thickness all over the envelope. A lumpy envelope with some thicker parts and some thinner parts will also be characterized as a parcel. Go to page 37 of the United States Postal Service's (USPS) price guide: https://pe.usps.com/cpim/ftp/manuals/dmm300/Notice123.pdf. This will tell you the amount you'll need to pay per ounce. You can use multiple stamps to reach the total amount or visit the post office to pay by cash, check, or card. It costs $0.98 USD to send a large envelope that weighs 1 ounce (28g). So long as your large envelope weighs less than 13 ounces (370g), you can send it by dropping it in a blue USPS mailbox or by leaving it in your home mailbox with the red flag raised. If your envelope is less than 618 by 1112 inches (16 by 29cm), it counts as a letter rather than a large envelope. This means you don't have to pay as much! Your letter can't, however, be thicker than 14 inch (0.64cm).
Use an envelope that's no bigger than 12 by 15 inches (30 by 38cm). Check that your envelope isn't thicker than 34 inch (1.9cm). Weigh your item to see how much postage you need to pay. Pay a lower price if your envelope is letter-sized.