Hi there.
I'm glad to see you in Community. Welcome! QuickBooks Self-Employed is a great product for capturing your transactions seamlessly. Ensuring you're invoices have been paid puts your mind at ease. I'll be happy to assist!
Here's how to manually receive payment on an invoice:
1. Open the Invoices menu and locate the desired invoice
2. From the Action column, choose Mark as paid
3. Select Send receipt from the ▼ dropdown in the Action column
Here's a helpful article for your reference, regarding invoices in QuickBooks Self-Employed.
Please don't hesitate to reach back out if you have any other questions. We'd be glad to help!