Ask HN: How much legal work is involved to run a non-profit startup?

I am planning on creating a web service where organizations caring for foster kids can post their specific needs ( something like, John's 7th birthday is on Sep 30 and he would like a skateboard with a product link to Amazon) and donors can pick a need and sponsor by paying the cost of the product + shipping. Once I receive payment, I will place the order on Amazon to be shipped to the kid's address and email receipt to the donor. My questions: What kind of a legal structure do I need for this? Does it have to be (or better be) a non-profit? How much legal work is involved upfront? Once created, what other (legal and accounting work etc) is involved on an on-going basis? Am I missing anything?