Upgrading your coilover suspension changes how your car handles every corner and sits in the driveway, but it is not a cheap project. A decent kit usually runs about $2000 once you factor in parts, professional installation, and the mandatory alignment that has to happen right after. Timing the purchase around when your tires are already worn out saves you a separate trip to the shop, since you will be paying for suspension geometry work anyway. Prices on parts tend to soften a bit during late fall and holiday promotional windows, making that a smart time to hunt for discounts before spring installation season hits.
Before you talk about payment plans, separate the actual price of the parts from the financing. Negotiation happens with the parts dealer or the shop doing the install, not with the bank backing your card. You want to hammer out the bottom-line price for the hardware and labor first, and only then figure out how you are going to spread out the payments so your bank account does not take a sudden hit.
Remember that the 18-month window on the Citi Simplicity card is a strict deadline, not free money. If you still have a remaining balance when those 18 months are up, deferred interest or standard rates will kick in on what is left, turning your smooth suspension upgrade into an expensive headache.