Charging Sales Tax on Price Other than sales price

We sell wireless phones, sometimes for $0. We are opening stores in RI and we are required to charge sales tax in RI on the price of the phone PLUS the commission we receive on the phone.

We do have transaction item options that a sales rep chooses when selling a phone. The item option is actually a referral to a custom record that has another field containing our commission for that transaction item option.

So, I need to charge sales tax on the phone's selling price ($10, etc.) plus our commission (from the custom item option record).

I need to find a way to make it easy on the sales rep, even for refunds.... I was thinking perhaps a script that runs to validate the line that then inserts a new line with a blank description and a hidden amount, but a valid tax amount. I just need to be careful and make sure reps can easily handle returns, etc.