I need to implement BrainTree Subscription Services into my existing web site

I have an existing subscription system that is able to receive money from PayPal and saves the payments and calculates how long they are valid for. I need to add BrainTree to this system. I can also completely replace PayPal with BrainTree if this is easier, but ultimately need to track a user with their subscription. The existing site is written in ASP.Net and uses the standard ASPNet account and roles. I'm open to replacing this with a different login system if this makes it easier as long as I can convert my existing information.