"With sufficient postage" just means you need to put a stamp on the outside of the envelope you are mailing to them - they are not covering the postage to get your rebate submission to them. If it said to include a "self-addressed stamped envelope", then that means you need to include return postage.
If anyone is looking for a form, I have a few extras. I can add one to any trade for free, or I can mail you one in exchange for the postage (1 stamp or $3 or so from my wishlist). PM me if you're interested :)