"There is a post office."
Translation:Es gibt eine Post.
What is the semantic difference between Es gibt eine Post and Da ist eine Post?
That would mean "A post office gives there." The idiom is "es gibt," not "da gibt."
You can use "es gibt" to mean "there is [something]," saying that the thing exists or is present, or you can use "da ist" to mean "[something] is there," saying that there thing is in some particular place, but you're mixing these up.
It's just a matter of putting the verb "gibt" in the right position for the sentence. The verb belongs second in a statement, and first in a question. So we have the statement "Es gibt eine Post" here, but if we made it into a question we would have "Gibt es eine Post?" Also if we wanted to put something other than "es" before the verb in the statement we could say something like "In meiner Stadt gibt es eine Post."