A sedan is a long-term commitment, and keeping the interior from cracking under the sun is part of the basic upkeep. A solid tint job usually runs about $400, though you can often find local shops dropping their prices during the slower winter months.
You should negotiate the price before you mention how you plan to pay. Once you have the final quote, you can spread the cost out to keep your cash flow steady while the job gets done.
The 0% period is a deadline, not a gift. If you still have a balance after the 18 months are up, the bank will start charging interest on whatever is left, which defeats the point of splitting up the cost.