Salt, mud, and months of trapped moisture turn your car interior into a disaster zone by the time spring hits. Keeping a car running well means dealing with the cabin too, and detailers usually drop their rates right after winter ends before the summer rush hits. A solid interior deep clean runs about $250, which gets you seats shampooed, salt stains lifted, and every vent cleared out.
You want to separate the price negotiation from how you pay for it. Nail down the final quote with the detailer first before you talk about plastic or payment terms. When you are ready to pay, spreading that cost out can keep your bank account from taking a sudden hit all at once.
That 0% intro window is a hard deadline, not free money. If you still have a balance left when the 15 months are up, deferred interest will hit you hard on whatever is left over.