Owning a car means signing up for a steady stream of small maintenance bills that keep the thing rolling straight. A suspension alignment and tire balancing job runs about $250 every ten thousand miles, and if you skip it, you end up paying for a whole new set of tires way too early. Service shops are often slowest right after the holidays in January or mid-summer in July, which is the best time to ask for a deal on labor.
You should always negotiate the shop bill on its own merits before you ever mention how you plan to pay for it. Get a firm quote on the alignment and balancing, ask them to waive the shop supply fees, and only pull out your payment plan after the price is locked in.
Remember that the introductory window is a hard deadline, not free money. If you still have a remaining balance on the Citi Simplicity® Credit Card when those 18 months are up, you will get hit with interest charges on the original amount.