The USPS has dropped its overall energy usage by nine per cent in the period between 2005 and 2008.
Overall usage dropped 13 million gigajoules (GJ), according to information on the agency's first ever sustainability report, published in the Environmental Leader.
The USPS also increased its usage of alternative fuels by 61 per cent and escalated the amount of recycled materials used by nearly four-fifths.
Last month, the service announced plans to cut the greenhouse emissions on their 220,000-strong fleet of vehicles, as well as in their chain of 34,000 postal facilities.
Paul Bastinelli, the postmaster of Eugene, Oregon, said that the service was committed to going green, with plans to divert paper wastage from landfills and retire up to 10,000 non-energy efficient vehicles from their fleet.
He said: "The Postal Service is serious about conserving energy and protecting the environment. On track to meet its stated goal of reducing energy use 30 per cent by 2015, the Postal Service plans to reduce petroleum use 20 per cent by 2015 and has increased alternative fuel use 61 per cent."