lemme just ask, have you ever looked at a mirror? --- Post updated --- also do you not know that you can just custom order mugs n shit and have them printed on with whatever you want?
Shirts : doable, there's ton of sites that allow you to design custom shirts Coffee cups : Great for tDev. Same as above. (as Modem mentioned) Wallpapers : git gud at art and done Scale models : Probably doable by modifying the car models and printing them with a 3D printer.
Making the items isn't the problem. Anyone could do that. It's getting official company licensing and mass production. Sure, it's cool and good that you ordered a BeamNG coffee cup, but that's nowhere on the scale of full on, mass produced, licensed merchandise, which I doubt will come soon due to the relatively small size of the company.
Some CAD programs even have an automatic "Fill Holes" function that would make it printable. I might try it when I have time for something other than life.