If you are mailing in a #000 bubble envelope, minimum postage you will be charged by your post office is $1.13 for up to one ounce. Each additional ounce will cost .17 cents. Since room keys are not breakable ... like chips are ... I would suggest using a plain white envelope. Wrap the keys in paper, or put the keys into a bank coin envelope, and scotch tape the folded paper or the coin envelope to the inside of the mailing envelope.

Hope this helps....