Auntie Anne's Soft Pretzels

I sent a very important document by express mail (overnight). Postage cost $16.50. The next day it was returned back to me stating POSTAGE DUE, Postage was on it. I tried calling the post office to inform them of this matter. All post office locations phones remained busy. I had no way of contacting them. I had to resend the mail through FED EX and pay postage again after the post office already got their postage for something they did not deliver.