We use ChargeBee for our SaaS product. Really great offering! They give you a customer portal where the customer can cancel, update, upgrade, ... his subscription. We have a custom checkout page where all options are listed. Then one API call and the user is forwarded and can fill out billing address, payment information, etc. on a ChargeBee page. They will handle taxes, issue invoice and everything else for you.

They have also an extensive API and webhooks.