1) Create a new payment form (or find the form you would like to edit.)

2) Find the Amount section and click "Yes" to answer the question, "Charge an additional fee?"

3) Choose "Optional" to answer the question, "Is fee optional or required?"

4) Click Save & Finish in order to save your changes.

5) Now, when your payers check out, they will see the option to add the fee to their checkout and can select the check box if they wish to do so.