I think a donation outside of buying the bundle would be nice, but I'm not necessarily the best person to ask about it. I'm just one person and I'm sure you got more than one other interested in your game. I know advertising can be very difficult.
Most charities get into quite a bit of trouble if they just let people do that without some sort of mandatory minimum donation and I found nothing about it on the submission rules. I should have contacted the website with my grievances and not jumped so quickly on someone. It was entirely unfair of me and I really do apologize(am canadian).
I hope you do too.