Add Promo Code to Paypal pro Integration

I would like there to be a promo code field on this page: [login to view URL] after 'Send me occasional emails...." check box. Valid promo codes will be : about 20 of them I would provide them Not all users will have a promo code so this will not be a necessary field unlike the other fields. The promo code entered will have to match one of the ones above so I guess there will have to be a check function on the backend? Not sure how that works. People who enter a valid promo code will be charged $19 instead of the typical $20, so on this page: [login to view URL] total will be $19 rather than $20. On the database side, I would like there to be a column that will record the promo code entered. No changes needs on invoice. additionally, if someone enters an invalid code, can we have an error message pop up in the same place at the top like the other error message when users forget to enter a field? Hope that makes sense. I want this completed in a day or 2 so only bid if you can start immediately and deliver it in time.