My two cents, both school of though have validity. The key here in the end is accountability. By driving the promotional discount to advertising it will allow one to easily monitor the success of the service marketing campaign, but at the same time actual gross profit on the sale would be overstated and it would likely lead to an overpayment of commissions bonuses based on gross profit.
I vote that a service promotion be treated like all maintenance menu items, with a predetermined operation code on the promotional item that the adviser would use (may need a couple) when they are generating a repair order that has a fixed Labor Price, a predesigned fixed Parts Package price, and finally a Job Description which creates true value into it which the service customer will read while reviewing the repair order billing.
This technique than would guarantee that all parties involve are participating accordingly in the promotional cost, controlled by the selling price and could potentially keep the sales tax man off your back when it come to the issue of coupons being taxable or not.
Additional points of interest are; if youre a GM dealer dont forget about that magical parts transfer, if I were a Parts manager I would not want to have an overinflated gross profit value used when calculating the parts transfer. As for tracking the success of the marketing program that could easily be done by monitor the number of times that specific operation code was used.