"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.