When your fuel pump goes, your car stutters and dies, usually right when you pull away from a red light. Keeping an older car on the road means swallowing these nine-hundred-dollar bills every few years, and they never seem to happen when your savings account is flush. Labor rates stay pretty steady all year, but independent shops sometimes run small parts discounts around major holiday weekends.
You should always negotiate the repair bill with the mechanic before you talk about how you are going to pay for it. Get a breakdown of parts versus labor, and ask if they can match online prices for the fuel pump assembly to shave a bit off the total.
Remember that the introductory window is a strict deadline, not free money, and any remaining balance left over when the 18 months are up will suddenly start accruing standard interest.