I bought a strength training machine (similar to a bowflex) online; and it was pricey. A few days after it arrived a second machine arrived. I checked my credit card transactions online and found that I had been charged for just one machine. But I really did not need two of these things. So I called up the company and explained what had happened. The company told me that they would send UPS out to pick up the second machine, at their expense naturally.
It is just courtesy to alert someone to harmful oversight (e.g. "Excuse me, don't forget your purse!").
It is also reasonable that people or companies absorb the cost of their mistakes (in this case pay for the retrieval of the machine).
It is not reasonable to take advantage of the company's mistake without making a good faith effort to correct it. You agreed to the trade of money for goods because you felt the goods were more valuable to you than the money. If it were otherwise, you would not have bought them! To claim a right to expoit the error because the goods were expensive is just a rationalization.
BTW, I'm still waiting for that second machine to be picked up
