I have no idea what the USPS compensates independent tow contractors. However if you are looking to contracting with various companies you should look into UHaul, Hertz, RGC Logistics, TQL, CoPart, IAA and etc. The more contracts at your disposal the more options you have instead of being limited to just providing roadside assistance. In addition, I would call around to various tow companies in your area and ask for quotes to get a idea of what the average tow costs including hook fees, miles to vehicle fees, loaded fees, and etc.