A smart motorized window shade system is a heavy investment that should last you a decade or more. Focus on the quality of the motor and fabric so you aren't replacing them in a few years.
Wait for the release cycle to shift. Manufacturers usually drop new tech in the fall, which means last year’s model often hits the clearance rack and performs just as well for a fraction of the cost.
That 18-month window is a deadline, not a gift. If you still have a balance once the time is up, the bank will start charging interest on whatever remains, which is where the math stops working in your favor.