That car of yours is a second living room, complete with the coffee spills, crumbs, and salt stains to prove it. Running a car costs enough in gas and insurance without a deep interior scrub catching you off guard. Spring is the sweet spot to book this because detailers are ramping up after the winter slump and prices tend to soften before the summer rush hits.
You should always negotiate the cleaning price before you talk about how you are paying for it. Get a firm quote from the detailer first, then decide if you want to pay cash or use a card to break up the cost.
That intro window is a hard deadline, not free money. If you still have a remaining balance on the Citi Simplicity® Credit Card when the 18 months are up, deferred interest or standard rates will kick in on what is left.