[Bruno85]

After you ship the package, please reply to this email with the tracking number. Having the tracking number on hand lets us issue your new order as soon as your package arrives at the warehouse.

The vulnerability here Is the tracking number. Do some research on (or If you already know)  the context of the tracking number, and see how It's formed- Identify the number and type of chars etc and formulate one that's based on a similar algorithm.

Then tell them you've already shipped the Item under a given tracking number. Obviously It will not exist, but that's the point of the SE. They cannot prove otherwise. As far as they're concerned, you've shipped the Item and given they can't locate It, hence you may well get a replacement sent to you. As with every SE, there's always an element of risk. Be aware of that.

I understand that, i did that with SteelSeries althought they didn't ask for the tracking number. I just said to them that i already sent the item and they believed it. Now with FitBit they didn't even send a shipping mail like with the bar code and everything, they just sent an address.
For me it's a bit tough because i'm still learning how to SE, i haven't done a lot of SE'ing, i read posts of methods people use but most of the times i can't do them. For example, the refunds i can't do it because my paypal account is still limited, i don't have a credit card to verify it. And the box method i'm still learning.