There is nothing quite like driving a car that looks freshly rolled off the showroom floor, especially right after you have paid for a professional detail. Keeping that gloss means fighting off bird droppings, UV rays, and road salt every single day, which is why a ceramic paint protection coating usually runs about $1200. Detailers often run their best specials in the late fall or early winter when shops are slower and you can actually negotiate a better rate on the labor.
Never mix up your negotiating strategy with how you pay the bill. Hammer out the final price of the coating and prep work first, then figure out your payment method for the deposit and extras instead of financing the whole shop bill by default.
Just remember that the intro window is a firm deadline and not free money. If you still have a remaining balance when the 18 months are up, deferred interest or standard purchase APR will kick in on whatever is left over.