Invoice optional fee

I want to create an optional fee that is added to the invoice. This would be a 2.9% credit card fee. This is optional from the admin side. In other words, there should be a yes or no box that is available in the add-invoice page. If clicked yes, a 2.9 fee is calculated and added to the total. Any ideas on the best way to do this?

Hello Dan,

Create 2 equation fields, 1 with 0+totalAmount value other containing 2.9% of the total amount+total amount.

and third numeric field which with conditional logic if a checkbox is selected then 2nd field other wise 1st field value

Sunny Singla