As a customer, what do I gain? As I see it, I take a significant risk in shipping with an unknown entity. The cost of shipping within the US is already fairly low, so why would I take the risk. You can't go international, even into Canada or Mexico, because I sure wouldn't take a package from an unknown person across the border for them.
You've created an opportunity for a driver with an empty truck, but you haven't (I don't think) really solved the question of why a customer would use the product. You say 50% off the cost is the benefit. But what is the average price of a product that can be moved in the manner you are describing?
If you are talking about getting door-to-door service, that is going to add up for the driver as well.
UPS, USPS, FEDEX, DHL, etc. all operate on a hub and spoke type system which is efficient with a large number of packages.
For local direct shipping, there are often lots of local options which may be cheaper than the big names. If you could link up the drivers across regional locations and offer them something so they can compete with UPS, etc, then you might be on to something.
An interesting annecdote, my great-grandfather got into the trucking business exactly as you are describing. He had a truck and was driving from town-to-town in Southern Ontario looking for work. People started recognizing him and asked him to take small packages for him on his travels. Then one day somebody asked him if he could deliver a Piano, and he clued in that this was a good thing. But that was in the early 1910's (I think), so there weren't a lot of shipping options.