Cars are expensive to keep. Between the gas and the insurance, the last thing you want is a salt-stained carpet that smells like old gym socks every time it rains. Custom-fit liners are the play here because they trap the mess instead of letting it soak into the floorboards.
The best time to buy is late autumn or early spring before the slush hits. You aren't negotiating a car price here, but you are protecting the car's resale value. High-quality sets usually run about $220, which is a lot for a piece of rubber, but cheaper than replacing the actual carpet later.
The 0% interest window is a fixed timeline, not a gift. If you still have a balance after those 18 months are up, the standard interest rate kicks in on whatever is left. Pay at least the monthly minimum to make sure the liners are paid off before the interest starts.