Owning a car is a constant cycle of maintenance to avoid bigger problems later. A transmission flush keeps your gears shifting clean, which is a lot cheaper than replacing the whole unit down the road.
You should expect to pay about $350 for this service. Check the dealership or local mechanic websites for coupons before you book, as they often have seasonal specials that shave a bit off the top.
Remember that the 0% intro window is just a temporary break. If you haven't paid off the $350 by the end of the 18 months, the bank will charge you interest on the remaining balance at their standard rate.