Collections
Collections is under Reports. The page title is Collections summary. It reads payments and workshop receipts for the shop you are signed into. Those receipts are internal documents, not tax invoices.
What was paid, and what is still due
The month table lists up to 24 months, newest first: how many payments were recorded, and the sum of those amounts. Beside that, the page totals the amount still due on receipts that are draft, issued, or partly paid, and it counts how many of those receipts are open.
A paid or cancelled receipt is not in that outstanding figure. Recording the payment itself is done on the receipt, under Operations.