A picture is worth 100 words. Below is a photo of Pennsylvania Avenue between 1750 on the right side and down to 1700 on the left side (the red brick building) You can see the post office at 1750 - it is just behind the low rectangular cement planter with the green grass in it. You cannot make out the details of the store at 1700 Pennsylvania Ave. N.W. in this photo, but the store at the end of this group of three buildings is a Wells Fargo.
I can see only two explanations:
1. As cjpalermo suggests, the post office was moved at some point to its current location. The second possibility is that the White House, which is just out of view further down the road on the right, may be leasing some office space on the upper floors of 1700 Pennsylvania for the purpose of using it as an internal office mailroom operation since space in all of the regular White House buildings is so tight. If that is where the internal mail room operations reside, they would not be a public post office and as such would not offer the services that a regular public post office would provide, any more than an internal mail room that every other part of the Federal government has for themselves in their buildings. If you want the post office closest to the White House that serves the public, that would be the one at 1750 Pennsylvania Ave. N.W. that is shown on the right side of the below photo.
