When I first got into the courier business, I had a standard pickup with the 8-foot bed and a cover. I ran that for a year or so, but the fact is that it's not the right vehicle for the job. It has the capacity of a station wagon and the fuel thirst of a large van-- so just get a van.
Can you do this in a pickup? Yes, but--- the bed is going to restrict you to the smallest loads--- seriously, if you can get a skid on there you're doing good, if the Avalanche is what I think it is-- and you'll find that those loads don't exactly grow on trees. Even in the small vehicles, you want something that can handle enough freight that they will call you (or, if you're strictly a one-man band and you're getting your loads on your own, you can find something to fit). That suggests nothing much smaller than the standard full-size van. Otherwise, you're going to sit a lot.