I had a similar problem which I resolved by contacting my Congressman, John Salizar.

Now I see that federal buildings and properties are not covered by the A.D.A.; of course these federal places are also not covered by state laws; however, service dogs are strongly covered by Title 40, section 3101 of the U.S. Code.
This has very strong wording for the disabled person's rights with service dogs in federal buildings and on federal property.
The Rehab Act of 1978 seems stronger than the Rehab Act of 1973 in favor of the disabled person's with a service dog rights...and especially addresses the US Postal Service & its properties: check 504 (A) under this.......
Access rights are preserved under the General Services Administration Federal Management Regulations.
It is possible that the ADA does not apply to federal property because disabled people with service dogs are previously and better covered by these acts and regulations.