Better read up on social engineering. What do you mean you need a serial number to social engineer one for yourself?
You need a S/N for an in-warranty Xbox for them to even consider offering to fix it. Then they would send you a box.
Idk if you know what the box method is, but its where you fu*k up the box to where it looks like someone stole it.
They get the box, ask wtf happened, you say it must have gotten stolen while in transit, Profit.