Donating Questions?

Hi. I'm currently thinking about donating, and I'm wondering what proof of donation needs to be shown, because I can't seem to find any listing of what needs to be shown. Also, with the "Unlimited NX", how is it unlimited (e.g. ask for top-ups whenever, truly unlimited or inordinate amount of NX, etc) and is this given for a limited time period or for an unlimited amount of time aswell? How soon the rewards are given doesn't really matter to me, as long as it's within a sensible amount of time (e.g. not 5 weeks).
Thanks, in advance.