Can small delivery companies afford the complexities of flat-bed trucks? Well: are they driving them? Repairing them? Assembling them? Loading/unloading?

You can bet that using a passenger car instead can be more "affordable", because more people know how to drive them, repair them, assemble, load/unload, they're cheaper, etc. However, if what they're delivering can't be hauled by a passenger car, or the logistics wouldn't make financial sense, that again changes the equation. There's no one answer.