Hello brian91,
In QuickBooks Online, receipts for invoice payments are not sent automatically. You'll need to sent it manually.
- Select Sales on the left menu and go to the All Sales tab.
- Look for the invoice payment. You can use the Filter if necessary.
- Click on the invoice payment and select Save and send.
- Select Send and close.
If you're referring to something else, additional details about this would be much appreciated.