They do not cover postage. They 'may' as a one off, but its best to be accurate. (See end of post)
Provide a return bubble mailer of sufficient size to hold 35-odd cards, with sufficient stamps to cover postage cost.
Im not sure if they all come back in top loaders, someone who has done a Predictor redemption would be able to tell that, and use that information to judge the overall weight, use Auspost website to calculate postage, and all should be good.
I misjudged the return postage cost for the 2012 AFL Screamer redemptions by a few dollars and got a nasty letter from Select in my package when it returned.