Owning a car comes with a steady stream of maintenance bills. Adding a cosmetic touch like painting your brake calipers is a cheap way to upgrade the look, but it still pulls $300 from your wallet.
The best time to handle this is when you are already swapping to summer tires. Since the shop already has the wheels off, they usually charge less for the extra labor. Don't pay the full price up front if you don't want to.
Remember that the 0% intro window is a deadline, not a gift. If you still have a balance after the 18 months are up, the bank will start charging you regular interest on what is left.