Do customers receive a receipt when an invoice is marked paid?

QuickBooks isn't capable of sending receipts when an invoice is marked as paid, @inthefieldrepair. Let me go into detail and provide an alternative option. 

To achieve this functionality, we can manually download the receipts and email them to your customers. 

Here's how: 

1. Open your QuickBooks Online account.
2. On the left panel, go to the Sales tab.
3. Under the Type drop-down menu, select the Money received.
4. Locate the Payment transaction in question.
5. Click View/Edit.
6. Select the Print option, then click Download on the Print preview screen.
7. After this, email the downloaded PDF to your customer.

Additionally, you might want to click on this article which helps you remind your customers about invoice due dates and ensure they pay on time: Send invoice reminders automatically or manually in QuickBooks Online.

I'm always available to circle back if you require additional assistance managing your invoices. It's great to have you here in the Community.