Even in commercial circles, You do not have to pay for the full rating of a truck. As long as you have paid enough to cover the weight of the rig if or when it gets weighed by the authorities, you are ok.
You can even lic. it for more than it's rated for. A buddy of mine has a Kenworth with a fixed load. The truck is overweight by about 3K# (33K# total) but the rigs lic. is good for that amount.
So when he gets caught at a weigh station, he gets a ticket for being overweight but not for being over his lic. rating - which are two very different offenses.