You are buying this e-bike to replace your car or transit hassle, so get something built to last daily weather and miles rather than the cheapest frame online. Paying once for a solid commuter beats buying a flimsy model twice in two years.
End of summer sales are the sweet spot because bike shops need floor space for incoming winter gear and next year's models. Last year's model with proven parts is almost always a better buy than a brand-new release with an untested motor.
Keep in mind that the 0% intro APR for 15 months on purchases and balance transfers is a deadline, not free money. If you still have a balance left when those 15 months are up, the standard interest rate kicks in on what you owe.