You ll have to readdress it, and put the full amount of postage needed. Instead of guessing, though, go to the post office and have them weigh it, and tell you how much is needed. Then you can pay the exact amount, and they can print out a sticker with that info on it.
In Australia you just make up the extra if the letter is returned but mostlly we send a bill for the difference plus $1.05 handling, but I guess it depends on the country you are in and their postal rules. Why don t you ask them?
I think 17 cents but to be sure check with the post office.