AFAIK it was next door to the embassy.
Now, if only Iran cared so much about embassy status, there wouldn't have been dozens of Americans held hostage in the US embassy for so long.
I am just trying to understand the "nitpick" part.
For the US government an annex building is considered a part of the building.
USPS has numerous government annex buildings.
An attack on an USPS annex building is an attack on USPS itself.