I got a request for a book and I printed the wrapper with the prepaid postage.  I then received another request from the some buyer for another book.  It would be nice to send both together and save a little (maybe) on postage but I've already printed the wrapper with postage on it.  Is there anyway to "undo" that?

If not, no big deal - I'll just send both separately.

you can't undo the printed wrapper.

however, i think it might be ok if you just added stamps to cover the extra postage needed to send the second book to your original wrapper.  (though i think if the package is then over 13oz, the rules for stamps apply and you have to hand it to a clerk.)  but check both parts of that, please.

Yep, just add the extra postage in stamps, or let the post office do it.

I'm doing that tomorrow, in fact!  I had a book going to someone and printed the postage, and then realized I also had a big box of books going to her too!