I'll be outside the post office, am I correct?
I'll be outside the post office, won't I?
Neither of these sentences are OK in either a formal or an informal setting, because the question tags are not appropriate to the preceding sentence.
Firstly, You use the words correct and right in a question tag to check whether information in the preceding sentence is factually correct. "Am I right/correct?" is quite hostile: you would use it if you think the other person disagrees with you, and you want them to acknowledge that you are actually right. If you simply want to confirm a fact, you would be more likely to say "Is that right/correct?" in a formal situation. "Right?" is only informal because you have omitted "Is that".
Second, you use these words when you are reasonably sure about the fact but the person you are asking will definitely know. This is therefore not appropriate when the preceding statement is about where you will be. You could use it about where the other person will be, but not about yourself, because you should know where you intent to/will be.
You'll be outside the post office, is that correct?
In this situation, you could also use the question tag "will you?" if you expect that the probability is low or you are skeptical or the statement is expressed as a negative. You would use "won't you" if the probability is high. If the main verb is not be, you can use the same trick with modals and have. For all other words the question tag will be based on do.
You'll be outside the post office, will you? - not likely or skeptical
You won't be outside the post office, will you? - negative statement
You'll be outside the post office, won't you? - likely
You can't drive, can you? - negative statement
You can drive, can't you? - likely
You smoke, do you? - skeptical
You don't smoke, do you? - negative statement
You smoke, don't you? - likely
You would only use a question tag about your own location if you wanted to check with the other person that it is convenient for them, or to check that they understand what the arrangements are.
I'll be outside the post office, is that OK? - convenient, informal
I'll be outside the post office, is that OK with you? - convenient, informal
I'll be outside the post office, is that convenient for you? - convenient, formal
I'll be outside the post office, have you got that? - understanding, informal
I'll be outside the post office, understood? - understanding, formal