Discount codes with PayPal?

I’m trying to create a form where the default price can be discounted with the user entering a code. I have two discount codes which result in either a 20 or 30% discount off the default price.
Do I use the calculation fields to calculate a cost based on which/if a code is entered? I only see room for one calculation no matter how many rules I add.
Do I do a calculation field for each code (or lack of code)? If so, how do I tell Paypal which field is the final cost?