Car door magnets can be a great way for a client to place their company name on the side of their vehicle for short periods of time. The magnets can be removed and then placed back on the vehicle if needed. It's important to not bend the magnets. They must be kept flat so that they do not curl. Once a magnet curls wind will grab it and it'll be lost. There is no guarantee magnets will not be lost or stolen. There are to many variables out of our hands that we have no control over.
The next option would be a sign the same size with an adhesive back. This will stick right to the door and easily last for 3-4 years. The cost is less then magnets and it will last longer, will not be stolen and we'll guarantee it does not fall off.
There is also a non-permanent adhesive for short term applications.