A side note about USPS.
Besides their standard rates you find at the Post Office and Online you can get "Commercial" and "Commercial Plus" based pricing.
Commercial is basically open to anyone using either Endicia, Stamps.com, or USPS.com Click-N-Ship.
But, if you ship more than 100,000 pieces of any one shipping method (such as First Class) you can get approved for Commercial Plus pricing which is quite the discount.
My motto "The worst anyone could ever say is no, if you never ask then you never know".