I may be late to this party, but I was able to print a transaction receipt by doing the following in QuickBooks Online:

1. After submitting payment, go back to the customer screen showing all invoices and find the invoice just paid.

2. Click on that invoice (not the blue links to the right) and a menu opens from the right side of the screen with invoice info (Open, Sent, etc.), and it should also show Paid and show a blue link to "View Payment #xxxx".

3. Clicking that link opens the Receive Payment page, with the paid invoice listed and checked, and just above will be a blue link "transaction processed".

4. Click that link to view the payment submission info, and select print to get a receipt. The formatting is awful, but it at least gives info to send the customer like processing date, transaction ID, etc.