A new car is a major commitment that stretches well beyond the sticker price. Between insurance, maintenance, and registration, those first few months are expensive. If you want high-end custom mats, wait until after you have negotiated the final car price so you do not accidentally roll the accessory cost into your long-term loan.
Dealers often mark up accessories significantly. By ordering them separately after you drive off the lot, you can shop around for the best fit and price without the pressure of the finance office.
That 0% intro APR window is a deadline, not free money. If you have a balance left on the card after those 15 months, you will start paying interest on whatever remains.