USPS spokesman Richard Watkins in explaining why window hours were being reduced in the  Chillicothe, MO. area said: 

As part of a nationwide program to reduce costs to their consumers, the U.S. Postal Service is shuffling some of its mail carriers around to different offices and condensing window hours at select locations.

Watkins went on to explain that nearly 75 percent of post offices across the nation are leased buildings. Because of the driving high number the economy has put on lease prices (well above the fair market value for these locations), the postal service must up its rates for those who ship through their specified windows to meet costs. By moving carriers to other offices and, therefore, using less space in certain locations, as well as by using buildings less with minimized window hour times, the postal service could be able to then ask for a lesser lease rate from the building’s owner (or force a move to another leasable location within the town that offers a lower price), in turn driving down the price of shipping for the customers.

source: Chillicothe Constitution-Tribune