Don't put the stamp there. Put it in the lower left corner, the lower right corner, anyplace but directly over the recipient's address. The rule is that there has to be postage on the front of the envelope, according to my USPS neighbor.

or break up the total postage into multiple stamps, and sprinkle them randomly over the envelope, and make the USPS cancel each one separately. I think they may even be on the back of the envelope, so long as the address can be read.