Your car takes a beating hauling gear, and adding a roof rack and cargo basket changes how you pack for every trip. Prices on gear tend to soften late in the summer once the peak travel season cools down, but you want your setup ready before the miles pile on. Running costs usually just mean a tiny drop in gas mileage from the extra wind resistance.
We can look at negotiating the price of the gear with the shop or online seller independently from how we pay for it. A financing window is there to smooth out the cash flow for the hardware and the installation extras, not to change the baseline price of what you are buying.
Remember that the intro window is a hard deadline, not free money. If you still have a balance left on the Discover it® Cash Back card when those 15 months are up, deferred interest or standard rates will kick in on what you owe.