How can the costs be too high vs using a service provider? Most likely every service provider will take a % cut, which, if i somewhat understand what you're trying to do, would be detrimental.

Why not get a VPS for a couple bucks, (EC2 = ~20$ p month), and use core RPC? I can almost guarantuee it will be less of a hassle than using something like coinpayments (which is not meant for this anyhow)