A truck bed is great until your gear gets soaked in a downpour, which makes a hard tonneau cover almost essential if you haul tools or luggage regularly. Between the cover itself and installation, you are looking at about $1200, and prices tend to dip a bit in late summer right before the rainy season rush hits.
Negotiating the price of the part and installation with the shop is a totally separate job from how you actually pay for it. Getting a good deal on the hardware takes some local comparison, while the financing is just there to smooth out the cash flow so you do not drain your checking account all at once.
That intro window is a hard deadline, not free money, and any remaining balance left over when those 15 months run out will suddenly start getting hit with regular interest.