If you are planning to advertise the quiz online (here), I'd suggest you ask for entrants' e-mail addresses. You can then notify winners by e-mail, to save postage. You can also elect to make the prize an Amazon voucher (or similar), and the price itself can also be e-mailed.
You also need to decide whether the quiz is aimed at ardent quizzers or at people just prepared to contribute to your charity. The former are quite likely to download a free quiz and then post it to you with their answers and entry fee. The latter are less likely to return the quiz, so it is better to get their entry fee up front before you release the quiz to them.
Following on from previous posts above, if people offer alternative answers to your questions, which on reflection turn out to be equally valid, I would say you should accept them. Otherwise you will annoy the entrants. If you find a mistake identified in your questions, you may have to exclude that question from the scores - of course, make sure you check the questions very thoroughly!
Best of luck!