I’m trying to figure out how my customer paid the invoice. No record of how they paid

I’m here to ensure this event will be settled, @gwyn-artz.

If you mark the invoice as paid in QuickBooks Self-Employed, it doesn't send a receipt automatically. You’ll have to select the Send receipt option on the dropdown so you have proof that you’ve received their payment.

On the other hand, if you have QuickBooks Payments (Merchant Service) connected, the invoice will automatically mark as paid once the customer pays online. You can track this payment on your downloaded bank transactions. Let me show you how:

1. Go to the Transactions menu.
2. In the Type dropdown, choose Income.
3. Filter the date period and the bank.
4. Locate the event.
   

Here’s an article about invoicing in QBSE. It contains steps to customize and process the event.

If I can be of any additional assistance please let me know by adding a comment below. I'll be sure to help you out.