Number 1 is correct.

Your sentence #1 sounds much better in English. You could also ask: "Where is the post office?" or "Do you know where the post office is?"

Thanks Peachey :-)

Only 1) is correct because we only use one reversal (as in a question form) per sentence. "Could you tell me" is your question, so the statement following the question word must have a normal word order. 2) is a broken sentence because you've started a new question without finishing the first question. Even without a grammatical analysis, 2) simply sounds wrong and disorganised.