。
It's passing right by a post office.
Literal
Post-office-of side [object-を] pass-[ongoing-てる]-[sentence-final-よ].
Ambiguous without context — subject is omitted. Could be 'I'm passing by,' 'you're passing by,' or 'the bus/train is passing by.' 通ってる is the dropped-い contraction of 通っている, describing an ongoing passage. そば ('side, nearby') is a general spatial noun. This would fit a scenario like 'look out the window — there's a post office right there.'