Get it, it's free, you should have received a steam key when you bought BeamNG, so you don't have to buy BeamNG again. If you have any issues regarding the steam key, Contact Support.
If you paid with Amazon it means you purchased BeamNG.Drive from Humble Store. You can either re-download the game from there (the Zip will get updated each time but it could take a bit longer) or just activate your Steam Key (Steam is free to use) and your game will be kept up to date automatically.