Legal Question in Civil Litigation in Maryland

You have no legal right to keep it and not pay for it, although depending on the price differential and their good will policy, the company may let it go. All the company "knows" is that you received a package with the order # they assigned to it. They don't necessarily know if their worker mistakenly put the wrong item in the package. I assume you prefer the item you received to the one you ordered. You can wait to see if they take any action, but if they discover the mistake, they may expect you to return it (at their expense) in exchange for the item you ordered, or to charge you for the difference.