The last project I worked on required integrating with clients&#x27; existing payment infrastructure, including merchant accounts, and processing their customers&#x27; payments for them in addition to billing them a small monthly fee (we didn&#x27;t take any piece of customer payments). We used Chargify. It was pricey, I think about $120&#x2F;mo to support ACH for a few hundred clients, but the API is solid and well-documented. They&#x27;ve got several employees whose job it is to work on the official libraries, as well.<p>I am no longer a Chargify customer because that project never really panned out. I don&#x27;t have any official relationship with them.