Hello there, @mdpaintingexpert.
I'm here to shed some light on combining invoices as one transaction in QuickBooks Online.
We're unable to combine invoices in one transaction. However, we can mark all invoices paid in one payment if this is for multiple invoices.
Furthermore, explore how to receive and record invoice payments in QuickBooks Online in this article: Record invoice payments in QuickBooks Online.
Additionally, customers may overpay you for a variety of reasons. When this occurs, you have the option of applying the credit to an invoice, refunding the customer, or entering the overpayment as a tip in QuickBooks. The steps is outlined in this link: Handle a customer credit or overpayment in QuickBooks Online.
If you require any additional assistance, the Community is always available. I'd be delighted to assist you again.