Translation:At the post office
"На" literally means "on", "у" literally means "in". We don't have any direct "relative" of "at", the closest would be "біля" which means "near" (but it's not always the same thing).
However, the same expression in English or Ukrainian involving either doesn't always translate one-to-one.
The English "at + [place]" is either на or у in Ukrainian, depending on the type of the place (+ exceptions). E.g. it's на пошті/роботі/стадіоні (at the post office/work/stadium), but у школі/парку/ресторані (at the school/park/restaurant).
Or if you're "at the building" meaning, you're not in it, you're waiting outside closeby, that would be "біля будинку".
From the point of view of grammar - yes. They mean the same. They are both grammatically correct.
It is advised though, for melodic speech, readability and other aesthetic reasons to use the rule of alternating vowels and consonants i.e. avoiding two vowels or two consonants "clashing". If the word after "у/в" starts with a vowel, use в (e.g. в Україні), if it starts with a consonant, use у (e.g. у Польщі)