Owning a car means constant maintenance, and brakes are the one thing you really cannot put off. When you hear that grinding, your rotors are getting eaten up, which turns a standard fix into a much more expensive repair.
Shops often have slower days mid-week, so call around to see who can give you a better deal for a Tuesday appointment. You are looking for a fair price on parts and labor, not a bargain that compromises your safety.
Remember that the 18-month window is a deadline, not free money. If you have a remaining balance after that time, the bank will start charging you interest on whatever is left, which defeats the purpose of the plan.