TIP: If your envelope can't fit through USPS mail processing machines, or is rigid, lumpy or has clasps, string, or buttons, it's 'nonmachinable' and you'll have to pay $0.40 more to send it. If your large envelope is nonrectangular, rigid (can't bend), or lumpy (not uniformly thick), you'll have to pay the package price. If you want to send letter-sized papers without folding them, you can use a large envelope (called a 'flat') the postage for flats starts at $1.35. 10 envelope is 9-1/2' long x 4-1/8' high.) You can fold what you put in your envelope, but it needs to stay flat-no more than 1/4' thick. Your envelope can be a maximum of 11-1/2' long x 6-1/8' high.
Envelopes must be rectangular and made of paper to qualify for letter prices.
