A good set of custom floor liners is one of those car upgrades you never regret once the slush and mud start piling up. Keeping your carpets clean saves you time and keeps the resale value up down the road, even if the upfront cost stings a bit when you add up the shipping and tax.
You want to buy right before winter or rainy season hits, when stores still have steady stock before the rush. Work out the price of the liners on their own first, and keep financing separate from any haggling you do on the product itself.
Remember that the intro window is a strict deadline, not free money. If you still carry a balance when the 15 months are up, interest kicks in on the original amount from day one.