We sell prepaid phone topup vouchers. Often, they are a fixed price and that's no problem. We also have ones where the customer tells us the dollar amount they want added and that number can be anything.
Would like the ability for that particular line item, to have the cost reflect a percentage of what that sells for.

Presently, if the top up is $50, it shows in sales reports as a profit of $50 (because cost is reflected as zero), where, in fact the profit is like $1.20.
If this idea is already possible, please direct me to where it is.