Owning a car is a steady drain of gas, insurance, and wear. Car prices and parts tend to be softest late in the year and over major holiday weekends when retailers want old inventory gone. Always keep negotiating the vehicle completely separate from financing the smaller add-ons. A card window is meant for the deposit and the extras, not the whole car.
A durable custom-fit sunshade and a full set of laser-measured floor mats usually run about $300 together. It feels steep for pieces of rubber and foil, but preserving your interior saves your resale value. Spreading out that small hit makes it much easier to swallow.
Remember that zero interest is a deadline, not free money. If you still have a balance when the 18 months run out, the card's regular variable interest rate applies to whatever is left, so treat that payoff schedule as non-negotiable.