I would love to see a feature added where you can send a customer one receipt for payments made on multiple invoices. Based on a conversation I just had with a ST representative the only "work around" is to send a statement, however, that only shows to customer open invoices from what I can see.
It is possible within the payment gateway to set up a user account to receive payment receipts whenever a credit card payment is processed. This however does not automatically send the receipt to the customer. Since ServiceTitan is an invoice based processing software, the invoice itself is the document which contains the customers signatures and is intended to be used as the payment receipt (instead of a direct Credit Card receipt).
I recommend you reach out to your payments team- email@example.com for more information!