WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
Stripe - Online payment processing for internet businesses. Stripe is a suite of payment APIs that powers commerce for online businesses of all sizes. Use Stripe’s payment platform to accept and process payments online for easy-to-use commerce solutions.
HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.
TrueLayer - Simple Bank APIs to access real-time financial data
Blesta - Blesta is professional client management, billing, and support software.
ACH payments with Stripe - Accept bank payments with a few lines of code.