I used to own a FedEx Ground route. It is a high volume, low profit margin business model. They need to keep the conveyor running. Guitar products tend to be too large for their systems, their systems are designed for boxes about 18" cube. Long boxes get stuck on turns and either fall off...
The choices are limitless.
At the Fractal Forum there are several guys using traditional cabs, and I'm pretty sure Leon Todd did a YouTube video about doing so.