A good electric bike is a car replacement, not a toy. If you buy a cheap one, you will end up buying a better one anyway in two years when the battery dies or the brakes give out. Invest in a solid frame and a mid-drive motor that will last you a decade.
The trick is timing. Bike brands release new models every fall, which means bike shops slash prices on last year's stock starting in September. You get the exact same frame and motor for hundreds of dollars less, just because of a sticker change.
Remember, a 0% APR window is a deadline, not free money. If you have a remaining balance when the 15 months are up, the standard interest rate kicks in on whatever you still owe, making your discounted bike a lot more expensive.