If you have a package in the mail that you did not order, it could be a scam.
The warning comes from the Federal Trade Commission and cyber experts.
They say the mystery episodes are known as scams.
Officials say third-party sellers on Amazon, eBay and other online markets are paying people to write fake, positive reviews about their products.
A fake account is used to place the orders to random people.
Then the brushes send a cheap, often lightweight article in place of the more expensive product being reviewed.
The FTC is behind marketers who use fake reviews, but it is not illegal to send unordered merchandise to customers.