A performance intake and filter upgrade gives your car a sharper throttle response and a deeper growl under the hood. Running a vehicle isn't cheap, between routine maintenance, fuel, and occasional upgrades, so keeping an eye on total costs matters. The best time of year to get this done is January, when automotive shops are typically slower and offer better discounts on labor and parts.
You should always negotiate the price of the parts and the install labor with the shop before you talk about how you are paying. A card window is just a tool for managing your cash flow for the deposit and the bill, rarely something you should rely on without a solid repayment plan.
That intro window is a strict deadline, not free money. If you still have a balance left on the Citi Simplicity® Credit Card when those 18 months are up, you will get hit with interest charges on the remaining amount.