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) 

This is what your invoice will look like:

Please note that "TAX" cannot be removed from the invoice. This fee also cannot be removed if a client pays by cash or check.

*Adding additional service fees for clients for using a credit card was illegal in the states of Colorado, Connecticut, Florida, Kansas, Maine, Massachusetts, New York, Oklahoma, and Texas at the time this article was written. Check with an attorney or on your local laws before adding this charge.*

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