A good window tint makes a huge difference in the summer heat and keeps your dashboard from cracking. Waiting until the temperature spikes is a bad move because shops get slammed and prices don't budge.
Book your install for early spring. Shops are usually looking for work then, which makes it the best time to negotiate a better deal on the labor before the rush hits.
Remember that the 0% intro APR is a limited-time window, not a permanent lack of interest. If you still have a balance after 15 months, the card will start charging interest on whatever is left, which will quickly cost you more than the tint job itself.