Yes. One or two coins is no problem, if the envelope is strong and doesn't break. How many is too many? Try it and see. I bet a handful would break the envelope, but a few would be no problem. The US Postal Service limit for a letter is 1/4" thick. If your letter is thicker you will have to pay extra, but the weight of the coins will cost extra already. What you should never do is mail a ballpoint pen in a letter. Apparently it's very common to try, and the sorting machines rip the envelope open by accident if there is a pen in it. The US Postal Service recommends that yoiu do not mail cash, simply becasue it could be lost or stolen (the postal service itslef is very good, but mailboxes sometimes are robbed) It is not forbidden to mail cash, however. If you do mail cash be sure to fold it inside a piece of opaque paper so nobody can see what it is through the envelope.

hey, Sophia, you can do it in China. i did for several times,and i even sent someone a letter with a card(i mean the card like a credit card ) in the envelope.