How to add service fees to my forms

Hi I was wondering how I could add the stripe service fees (2.9%+$0.30) to both of my forms. Currently, on one form, I have the tax field set up to include the 2.9% but I'm not able to add the $0.30. One the other form, since it is a donation form where users input a number, I'm not able to add a tax field. Is there some way to solve this?

Nevermind figured it out.

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