Great game! You done a great job implementing the theme and also the special object so well together. I really liked the main mechanic of the game it felt like Hill Climb Racing mixed with Totally Reliable Delivery Service. However I will say that it felt very random sometimes how the boxes would would react to bumps and jumps and you kind of just had to hope and pray that the boxes would play nice. Also I started with trying to be really careful and would try to plan and take it easy going over every hill and bump. However it felt like sometimes it was a better strategy to just floor it to the end. So the player wasn't really rewarded for taking it easy and planning carefully.
I also wish you would have been able to zoom in and out as I wasn't able to see my second trailer fully so I was kind of just hoping that the boxes were behaving back there although I really wasn't able to tell. Obviously some music and sound effects would have really tied the game together although I know it can be hard to find time for everything in a game jam.
Overall great game and it has plenty of potential to be expanded upon. It would be cool if you could upgrade your engine, tires and trailers. If there was other types of terrain like mud, sand or snow. Also I know this is very different from you have made but it would be cool to have to create your own vehicle like in Bad Piggies and use it to deliver the crates throughout the level.