Owning a car means bracing for those random, expensive fixes that pop up out of nowhere. Catalytic converter theft is a real headache, and replacing a stolen one easily runs past two grand. Prices for anti-theft shields and labor tend to dip a bit when shops are slow, usually right after the holidays in late winter.
You should always negotiate the price of the part and the installation labor before you talk about how you are paying. A card window is just a tool to smooth out the cash flow for the bill, not an excuse to skip haggling.
Remember that the intro window is a deadline, not free money, and if any balance is left over when those 18 months end, you will get hit with back interest.