How do I cancel a trade-in? What happens if I don't send the trade-in device?

Updated 

If you change your mind or forget to send in your trade-in device, there's no need to take any action—the instant credit will be taken off the order and will be charged to your original payment method.

The charge will occur after your order's trade-in window has passed, which is 15 days from when you receive the shipping label.