I remember this happening to me long ago.
I freaked out, because it was a hard drive and back in the day, a big HDD was expensive.
The place in question made a plan for me, they told me I just need to provide proof I spent the money with them, so I went to FNB and got a statement of paying with my debit card and they accepted it.
Remember, all places generally keep stock numbers / serial numbers of the items they sell and many others keep copies of the invoices / slips.
You just need to find the person there who is willing to go outta the way for you and match the sale to your money spent.
If you paid cash, I'd suggest calling the store NOW already and trying to organise a slip, the date you purchased it, the time ± you bought it, etc, you know... help them to help you.
Also, maybe contact Microsoft XBOX distributor and ask them for info on this.
There is always someone out there to help you, just have to find them / have the will to find them.