An LED bulb runs a charge through the light, from one point to another and doesn't need a negative wire. A polarized bulb has a + and - pole (socket input), so you have to insert the bulb in a specific direction depending on how your fixture is wired with a positive and neutral wire. A non polarized LED uses a 4 diode bridge rectifier so the bulb can be used in either 12V AC or 12V DC and it doesn't matter which way they are put in.
I have superbrightled's in all of my sockets. They are non-polarized so they go into any 921 socket.
Why they want to send your back, is all you need to do it insert it again and if it doesn't work, flip it over and the positive and negative will be correct.