Radio Button Script help

Hello, I only know the basics of Java coding so I am not very good.

I have a document I am working with that has cuustom form fields.

I need help writing a script that says. If "choice1" is checked then it will discount 5% from the "Grand Total" field. And if "choice2" is checked then the "Grand Total" field will stay the same.

I am using radial buttons.

the "Grand Total" field is calculating the sum of multiple fields also.

so essentially if a customer wants to click yes to pickup they get a 5% discount from their total.