Recommendations for a subscription service?

Hey all. It's finally getting real for me! After getting inspired by a super-old (now deleted) reddit forum, it looks like my small web consulting agency (I think that's what it is) is finally starting to become a reality.

But I'm gonna need a way to bill customers. And most likely I won't need a "customer portal". And probably many customers will pay in cash. So looking for recommendations.

FYI, the business is a subscription-based website service for the small business immigrant community. Unfortunately, this market is traditionally low tech and may be even be "unbanked", so I expect many customers to pay in cash. For similar reasons, a SMS interface is ideal.

So looking for ways to bill and manage customers. Probably looking at a few dozen customers since I intend to keep this business small.