A good ceramic tint is one of the best upgrades for a daily driver. It keeps the cabin temperature down and protects your interior from fading, which makes the car much nicer to live with when the temperature spikes. Prices usually hover around $500 for a full professional job.
Shops are often busiest right when the first heatwave hits in late spring. Try booking your appointment in the early spring before the rush. You might find a shop more willing to negotiate on the price if you schedule during their slower weeks.
The 0% intro APR is a window to manage your cash flow, not free money. If you have any balance left after the 15 months are up, the card issuer will start charging interest on whatever is remaining.