Fee field dependent on answering "Yes" or "No" in another field

I would like to add a field to charge 2% for Paypal or credit card charges. But I don't want it to apply automatically to every payment through the payment plug-in, I want to provide users with the option of voluntarily paying it.

The payment should be based on a list field named [RENEW_TYPE] that is the type of membership they are renewing e.g. Adult $525, Family $1050, Student $150 etc.

I have a checkbox field with the options "Yes" or "No" (i.e. to volunteer the payment).

Then I have a fee field named [SURCHARGE] that should be dependent on whether they answer "Yes" or "No", and with a fee formula of [RENEW_TYPE]*0.02.

However, I can't get the Surcharge field to accept "Yes" as the "depend on" option. If I tick the "Yes" box, it refuses to save it and just leaves it unticked.

How do I do it correctly?

I have fixed it, there was a comma in the "Yes, I agree" option that it didn't like.

My question now is how do I place the value of the Surcharge on the form for payees to see, rather than just adding it to the total amount?