As of today (1/31/23) this is how you get a receipt to a customer in QBO: 

1. Open Sales/Customer and click on the customer you need to send a receipt to.

2. Open the payment you want to send a receipt for.

3. Click Save and send in the drop down list on the bottom right.

4. A receipt will pop up and you can send the receipt with all the invoices that it was applied to.

Good luck,

S.