Is paying two invoices for the same vendor at onetime on QB online Bill Pay considered one or two transaction charges?

Welcome to the Community, @fjmcw. I can help you understand how the charges in your Bill Pay work. 

Each transaction processing incurs charges. In your case, even though you are paying both invoices simultaneously, each invoice is processed separately, which leads to two separate transaction charges.

To better understand how Bill Pay works, please refer to this article: Learn about QuickBooks Bill Pay.

Please post your questions on our Community forum for further assistance about the program. We're always happy to help!

@fjmcw 

You can signup for a Melio account for free to integrate with QBO and pay multi bills for the same vendor for free with ACH or a single fee with check.

https://affiliates.meliopayments.com/quickbooks

You can also get $100 as cashback.

https://affiliates.meliopayments.com/cashback