To prove your idea you can use paypal . . . the api isn't as nice put it's a great start to prove your idea/app.
I would recommend automating signup so that is seamless . . . but you could have your backend email you for cancelations, etc and do those manually through paypal.
In the beginning keep things simple and be willing to do things manually behind the scenes to keep time/costs low till you have signups and users getting value out of your app that will be subscribers long term.
If you go with PayPal I'd recommend only using their free services (e.g., the "subscribe button") or going with BrainTree (https://www.braintreepayments.com) which is also free and a nicer way to integrate PayPal than their paid APIs.
Payflow Pro by PayPal ($30/mo) is ridiculously complex and it's better to buy an additional 3rd-party service built on top to make it easier (you'd end up paying $100+/month plus 2.9%).
I would recommend automating signup so that is seamless . . . but you could have your backend email you for cancelations, etc and do those manually through paypal.
In the beginning keep things simple and be willing to do things manually behind the scenes to keep time/costs low till you have signups and users getting value out of your app that will be subscribers long term.