Taking your car on real trips means buying gear that gets beaten up by rain, dust, and heavy loads. Gear like crossbars usually sees its lowest prices in early spring when retailers clearance last year's stock before summer road trip season hits.
Negotiating the price of accessories works just like buying the car itself. Lock in your discount on the gear first, then figure out how to stretch out the payment without adding interest fees to the final bill.
Remember that an intro window is a fixed deadline, not free money. If you still have a balance on the Citi Simplicity® Credit Card after the 18 months end, regular interest charges kick in on whatever amount is left.