We'd like to be able to calculate a processing fee based on a formula so that we can pass on the cost of cc processing to purchasers. Any ideas on the best way to accomplish that?

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See answer below...

To answer the question:

Adding processing fees at checkout is probably against the law in your country.
At least in switzerland it is forbidden since Aug 2017.

It is most certainly against the terms of CC companies.
https://www.paypal-commu...ee-at-checkout/td-p/4204 

Hope that helps
Regards
Reto

To echo Reto's response, I'm not aware of any credit card processor that allows you to (legally) pass along credit card processing fees in the way you suggest. If you're caught, they'll permanently cancel your account.

Instead, you should incorporate these fees as part of the costs for the business overall and increase your product prices as appropriate to cover the costs of running the business.

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