Customer Portal
All invoices sent via email should have a link embedded that when clicked will bring the customer to a page that shows their entire billing history. The customer should also have the ability to pay off their entire account balance on this page as opposed to just an invoice.
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Wayne Higdon commented
adding to the above: The customer should be able to log in to their portal at any time with a user name and password. The portal would have a history of past payments as well as the current unpaid invoices.
If the customer is also a QBO user, it would be great if this portal was integrated into their QBO vendor - so that a link in the Vendor Center for that vendor would jump you into the portal for that vendor.