Depending on your shipping volume, you'll likely find better rates through a USPS consolidator, like DHL.

A consolidator works in conjunction with USPS. They typically pick up mail at your location, deliver it to their nearest shipping hub and then deliver each piece to the USPS location closest to the shipping destination. The mail is passed off to USPS and USPS takes it from there, delivering to the customer.

UPS and Fedex (I believe) also offer similar services.