When an invoice is sent and paid through QBO and the quickbooks payment fee is charged, does it include GST?

The $7.50 is solely a processing fee, qboadmin.

It does not include GST or any other tax. You can read more details about the fees in this article: Intuit payments pricing schedule - Canada.

You can also check out these articles as well for additional guidance when accepting online payments from your customers:

1. Find out when QuickBooks Payments deposits customer payments
2. Change the deposit bank account for QuickBooks Payments

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