Scam.
I've read about this.
Paypal states that under no circumstances are you to reship the product unless they pay you for it. If it was a real error then they should be willing to pay for it to be reshipped. If they're not then they'll probably try to get their money back through Paypal, leaving you without product and without pay.
Be careful with this one.
Ask them to pay for the item to be reshipped. But also think: was your address on the back of the package? If it was then it should surely be returned to you. Personally, I'd wait for a week, then reship it, explaining that you'd like to give the parcel the chance to return to you before replacing it so that you don't lose money yourself - you're a business, afterall.
3 weeks is an awfully long time to suddenly say you changed your address. It's the sort of thing you notice after a day or two, not 3 weeks.