A good electric bike turns a sweaty commute into a smooth ride, but you want one built to last so you aren't shopping for a replacement in two years. Look for a sturdy frame and a motor system from a known brand that local shops can actually service.
Wait until late autumn to buy. That is when shops clear the floor for next year's models, meaning you can often find a current-year bike discounted by a few hundred dollars. If you buy during this window, you get the same tech for a better price.
That 18-month window is a strict deadline, not free money. If you have any balance left after 18 months, the credit card company will start charging interest on the remaining amount at a much higher rate.