I think with Market Force, rescheduling a few times is not bad, otherwise why do they give that option? It is canceling a shop without a valid reason that would really affect your record. After working for MFI for many years, I just canceled one shop because of conflict with CFA shops which they called me about. There was no other date to reschedule it. But they still gave me a chance to give my reason. Your rescheduling the shop will be safer, I bet.