I have been allowing a free download for a while for this very reason, and I will also email a set of answers to entrants if they would prefer to receive them that way. At the moment I am still only accepting entrys by mail as I haven't found a way of collecting entry fees electronically that I am happy with. Just Giving like the other sites do make a charge which is frequently funded by reclaiming Gift Aid, however the advice I have received is that you cannot claim Gift Aid on a quiz entry as it is not a pure donation, you are selling a quiz.