I am not sure if you have viewed Syncro rmm services for MSPs but they have several billing enhancements that would be very useful if added. One of those enhancements includes credit card payment integration with companies such as Stripe and Paypal. They allow you to post links in both the email invoice and on your website for your payment portal for the customer. Their invoice portal also shows when an invoice is overdue etc. They even have a timeclock feature that is added to keep track of your and your teams individual time working. They also have a service that will auto send out campaigns/notifications to clients regarding updates/services/end of life for products etc.