No idea on preventing it in the future, but if you are concerned about problems with using the envelope again, you can cut a a piece of paper to the size of the front of the envelope and tape it in place using packing tape. I did that after we moved... put my new mailing address and return address on it before taping it down, then covered the whole thing with strips of packing tape that went around to the back side. That envelope has been used from our apartment in NY to our house in Louisiana, and now our house in Oregon.

dp

yes, i agree with using alcohol to remove the marks, or StazOn stamp cleaner or GooBegone

Thanks everyone!

alcohol worked amazingly!!!

Also, use a dry erase marker and scribble on the cancellation marks. Then use the dry eraser and they come right off.

Tape a piece of paper over the extra postmarks. particularly if it hit an untapped area.