Owning a car is a constant battle against wear and tear. Between the daily commutes and the seasonal salt build-up, the upholstery takes a real beating. Getting a pro to handle the deep cleaning and leather repair is the best way to reset the clock on your interior.
Early spring is your best bet for a deal. Shops are often looking to fill their schedules after the winter lull, so they might be more willing to negotiate on the $500 total cost if you book before the summer rush hits.
Remember that the 18-month window is a hard deadline. If you have a balance remaining when the period ends, you will be hit with interest on whatever is left, which makes the service much more expensive than you planned.