jwolthuis - 2:59 am on May 15, 2011 (gmt 0)
The United States Postal Service has been prohibited by law from giving discounts to large shippers...
Not true for package services (aka, competitive services). Large shippers enjoy Commercial Plus pricing for package services, and can ship via first-class parcel up to 16 oz (rather than 13 oz).
This pricing level is available to shippers who meet established volume requirements (6k Express Mail pieces/year, or 100k Priority Mail pieces/year). They can call a hotline to sign-up.
You may be referring to their market-dominant services, such as first-class letters. That is their monopoly to enjoy, and prices are the same for everyone.