User Upgrade Pointers


Well-known member
So I'm making this modification, that will allow the administrator to add a list of coupons to. The coupon will be unique and would equate to a percentage value that would be knocked off the upgrade cost. So I have some questions I was hoping you guys could help me with:

I have an input field on the user upgrades field like so:

Now I was wondering, if I entered a coupon that was worth 20% off the price of the upgrade, how would I get that to process the new amount and use that for the new cost thus only allowing the user paying that much. Do you know how I could do that? If so could you provide me with some guidance?

Thanks in advance.