Yes, you can, although it is not a secure solution, you can.

Just use some cares:
put the bills inside two or more sheets of paper so that the bills are not detectable by looking at the envelope in intense light and also by touching it.
Use written sheets, so that the bills design are mixed up by the writings.
Send the envelope with some kind of postage insurance (also fast delivery/air mail work fine).

Thank you,
Nick.