Cancel it.
I needed clarification on a shop from Market Force - my first with them, and a last minute phone call asking me to please do - and did not hear back before I had to leave for the shop. I made the executive decision to take an educated guess and do the shop. Made the wrong guess and was out gas, two hours, shop fee AND the mandatory 15ish purchase, which was not at a place I would have made a purchase otherwise. I should not have taken pity on them. You will be doing yourself a favor to cancel and only work with responsive schedulers.
(And no, I have not accepted any other Market Force assignments since. They would have to be RIGHT next door for me to try them again, now.)
LA