Surcharge for Customer Accounts

I have items that are not discountable, and I have set that option under item>options>item is not discountable at the POS. But the thing is this, when a customer who has an account with us and has a discount, purchases one of the non-discountable item, I want to jack the item price by their discount. (Ex: retail price $5.00, customer's discount: 2%, price for customer $5.10) When I print the statement out at the end of billing cycle, if they pay by 15 days they only pay the retail price of the non-discountable items, but if they pay later, I want to charge the new price scheme. How do I do that