ALL versions are correct but (may) convey different meanings...
Max kommt aus der Schule:
Two different meanings:
Max just left the school building. You can see him coming out.
Max comes (home) from school.
Max kommt von der Schule:
Is more a direction: he comes from the direction where the school is.
So can mean the same as the first one:
Max comes (home) from school
BUT: Can also mean: He works for school.
Would be in a sentence like:
Max wants to see how our son is. He comes from his school.
AUS is usually associated with a building that you leave.
VON means the general direction where you come from.
In case of POST there is another little twitch:
AUS der Post usually means: Picking it out from between the other letters (probably out of the post- box)
VON der Post in that case means: she was at the Post- Building and got her package there.