MarketPress coupon code to increase price

We're using MarketPress with A+ to book lessons.

Booking in blocks of 12 is quite popular, but since A+ doesn't allow multiple bookings in a single go (say every Wednesday at 3pm for 12 weeks), we'd like to create a coupon code that will INCREASE the price by multiples, so that when the service provider sees it used, they (or an admin) can manually block off those dates.

Out of the box, the coupon feature doesn't use negative amounts, so were wondering where the underlying math is, and how easy it would be to create a "price increase" coupon.

For example, if a service providers 1 time fee is $70, if the customer wants to book 12 sessions, then we'd like to apply a multiple of 11 so the total is $770 (because the 12th visit is free).