A good set of custom fit seat covers makes an older interior look sharp and protects the upholstery from daily wear, spills, and pets. Keeping a car running well takes enough cash on maintenance and gas, so you want to time your upgrades right. Holiday weekends like Memorial Day, the Fourth of July, and Labor Day are usually when manufacturers and retailers drop their prices on interior accessories.
Separating the negotiation from how you pay makes the whole process easier. Haggle or hunt for discount codes on the product itself first, and only look at payment terms for what is left over rather than funding the whole purchase through credit by default.
That 0% window is a hard deadline, not free money, and any remaining balance left over when the 18 months run out will get hit with standard interest rates.