I have an international customer who would like to pay their invoice using ACH. However, my invoices only allow United States bank accounts. Maybe a setting I can change?

Consider this payment service to deal with multi-currency transaction. It supports QBO.

https://veem.grsm.io/veems

Hope it helps

Thank you for posting here in the Community, @lisa92.

Paying the invoice using an International bank is currently unavailable in QuickBooks Online (QBO). As for now, our Merchant services accept only the US bank. Nothing to worry, I'll personally send this feedback to our developers so they might consider adding this feature on any future updates. 

You can visit our Quickbooks blog for any product updates and feature enhancements.

As a workaround, let your customers use other apps like PayPal or other apps that are supported in QBO. Go to this link: https://apps.intuit.com/app/apps/search?searchTerm=ach. In this link, you can see the approved apps that you can use in paying invoices through ACH. 

For future reference on how to process an ACH bank transfer for your customer’s sales receipt or invoice in QuickBooks Online, please check this article: Process an ACH bank transfer from a customer.

Please come back if you have other concerns. I'm always here to help. Take good care.