I can share some information about customer payments, @qualomics.
The invoice are automatically marked as paid once you receive the customer payments. You can go to your invoices page to check for invoices that was marked as paid. You can see Paid under the Status column.
For future reference, let me share the create invoices in QuickBooks Self-Employed guide. It contains a breakdown of articles on how to add, manage, and send invoices.
Please know the Community has your back. If you have follow-up questions with your invoices or any other concerns, post a comment below. I'm here to assist you anytime. Have a good one!