Upgrading your suspension changes how the car handles every corner and bump in the road. A full set of custom performance shocks and struts runs about $1200, and that is before you factor in alignment and installation labor. Spring is usually the best time to watch for manufacturer rebate events that can shave a bit off the total cost.
We need to separate haggling over the parts price from how you actually pay for it. Get your best price from the shop or parts dealer first, and then figure out the payment plan. A financing window is just a tool to spread out the blow, not an excuse to spend money you do not have.
Remember that the intro window is a deadline, not free money. If you still have a balance left when those 18 months run out, the regular interest rate kicks in on what is left.