I would let them know of the mistake. The fact that they haven't discovered the mistake probably means they do a lot of business and didn't need them for someone else. Unlike someone like ourselves that may of had two wheels and quickly discovered the mistake when they realized they saw the wrong wheel in their garage or needed to send it to someone else. It is also possible if they are a larger volume seller that they didn't have the wheel in question and decided to send a better wheel thinking they did the "right thing". In any case you are in control and if they want the wheel back make sure all of your expenses and inconveniences are covered and possibly wait for the correct wheel to arrive before sending their mistake back. As others have said it is the right thing to do to notify them and is good kharma.