In short, the rate structure could be established in order to be competitive or to base the prices on the actual costs of the mail, and worksharing activity could occur as a natural result.
Rate structures are rigid and inflexible. 
mnli hypothesis: Rate structures are rigid and inflexible.  premise: In short, the rate structure could be established in order to be competitive or to base the prices on the actual costs of the mail, and worksharing activity could occur as a natural result.