If a retailer sends you too much stuff ...

... you don't have to return it.

To put it simply: you can keep it. According to the Federal Trade Commission, you have a legal right to keep unordered merchandise and consider it a free gift. That’s because federal law prohibits mailing unordered merchandise to consumers and then demanding payment...
In general, though you’re not legally obligated to tell the seller, if your conscience is pushing you in that direction, the FTC suggests that you notify the seller and offer to return the merchandise, so long as the seller is the one who will pay for all of the return shipping.