Can I put a large 12"x15" envelope in a USPS mailbox?

Question

it is only 2.5 ounces. I put about $1.60 of stamps on it.

Answer 1

i would just carry it to the main post office in your home town and give it to a carrier and they will mail it. It does not cost any more or less to do this and you will know it was sent

Answer 2

YES!  If your envelope weighs more than 1 oz. or is being mailed outside the United States - or if it's larger than 6 1/8 inches by 11 1/2 inches by 1/4 inch - additional postage will be required. Use a small kitchen scale or postal scale to weigh the letter; you can find the correct rate at the U.S. Postal Service Web site. If you don't have a scale, take the letter to the post office to determine the correct postage if you're not sure.