1.  Go to Settings > Services & Fees 

2. rename the Tax area  "Credit Card processing fee" and add it in there. 

3. add a note to your invoice that says "if you don't pay by credit card disregard the fee" (Settings > Payments) 

4. If they pay you by cash or check, when you go to add the payment to the Fees area on the inspection details page, you'd delete the tax off the invoice. 

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