Automatically Approve Users

I'm just looking more for a general direction here. We have users that sign up, and right now we have to go into our software where they purchase a subscription, validate they purchased it, then manually approve them. I already know how to talk to my purchasing software, and I'm wondering if it's possible to use the API to approve users based on an email address.

I don't necessarily need code, just if someone could point me in the right direction, like to the proper end points, I could go from there.