Billing for SaaS or Cloud service

What kind of billing system are you using for your cloud/saas solution? Have you developed it from scratch as a part of your product or adopted some existing framework? Or use a 3rd-party solution such as Stripe for that? Maybe you could recommend some just not to invent the wheel and minimize efforts for managing several product plans with recurrent payments, discounts, etc. I'd prefer to have the charging process outside the billing itself, just to keep it as secure as possible. Thanks!