Today I got a phone call from a very nice USPS representative, saying the problem was caused by my accessing my account from outside the USA. The rep (a lady) authorized my foreign IP number, and said I should not have any more trouble.
In sum, if your account is blocked for postage purchases, there must be a reason (in my case, my foreign IP). Other causes might be a mismatch between the billing address you enter for the card, and the bank-recorded billing address.
Update:
Today I got a phone call from a very nice USPS representative, saying the problem was caused by my accessing my account from outside the USA. The rep (a lady) authorized my foreign IP number, and said I should not have any more trouble.
In sum, if your account is blocked for postage purchases, there must be a reason (in my case, my foreign IP). Other causes might be a mismatch between the billing address you enter for the card, and the bank-recorded billing address.