You have up until a month for Amazon to receive the item to receive the credit. Regardless, locking in doesn't hurt you, you don't have to mail in trades just because you lock them in.
Oh. Are there any penalties for sending it in late? I wanted to wait because it was going to take a week to get delivered. I wanted to have enough time to get them to the ups drop location. The trade in option came back up so I got lucky this time.