What membership software should I use for a web application?

I am curren开发者_Python百科tly the programmer for an not-for-profit organization which specializes in tree restoration / planting trees.

Right now, we are developing a web app for our customers where each client can view its current purchases, the amount of trees that they have bought, and the exact areas where each tree is being planted.

What we're looking for right now is some sort of membership program to tie it all together. So we're looking for something that will be flexible enough to integrate our web application into while also having pre-integrated payment collection.

We would really appreciate any and all help!

Thanks, John