jwolthuis - 1:58 am on Oct 19, 2012 (gmt 0)
From the FTC's website
That applies to unsolicited receipt of merchandise, not a mistake trying to fulfill an order.
If the customer is in possession of your widget, file a small claims in his jurisdiction, add the cost of filing to your total damages, and let the judge decide if he can rightfully keep your widget.
It was shipped in error, tell the judge you corrected the mistake, and he's trying to take advantage by ignoring you. You have a case.