Welcome to Brazil the Land of Mediocrity. I know exactly how you feel, because I've had more than my share of troubles with the incompetence of Correios. Over the past year, no less than 6 of my Canada Pension Plan retirement pension checks have simply vanished from the face of the earth. As you might have already guessed, the only way to obtain a replacement check is to phone Service Canada, request they send out a request form (by mail), fill it out and send it back (also by mail). This process takes at least 6 months for each missing checki and Registered Mail between Macaé - RJ and anywhere in Canada takes at the very least one month to arrive, more often closer to two months. Of course NOBODY at Correios accepts responsibility for anything, and even trying to get compensation for any major financial losses they may cause you takes a civil action in the Tribunal Federal Regional (Regional Federal Court).
Correios delayed my 2013 Income Tax Return until well beyond the filing deadline for international returns, and as a result even though it was posted before the deadline Revenue Canada denied my Part 217 claim, which is now under appeal.
If you've been savvy enough to retain the receipt for mailing your income tax return, then have a certified copy of it made at the nearest Cartório and send it to the IRS with a full explanation that although you posted the return prior to the deadline it has been lost/delayed by Correios. That should at least protect you from any late filing fines imposed by the IRS. I would highly suggest that, given the circumstances, not trust that it will even arrive and complete and send in another 1040 just to cover yourself. Try FedEx and on the airbill indicate that the contents are DOCUMENTS because if you indicate that it is a letter or correspondence of any kind they likely won't accept it, because it will get them into hot water because of the protectionist laws here in Brazil that favor the incompetent Correios.
Go back to the agency where you posted your income tax return and demand a full refund, since they've lost the article. Even if they haven't actually lost it, they did not deliver it on schedule and that alone entitles you to a refund. Don't take no for an answer.
I don't know if US citizens have the option to e-file international tax returns, but if they do, that is what you should be doing in order to keep yourself on good terms with Uncle Sam. Sad to say we Canadians don't have that luxury, we are forced to paper file international returns.
Cheers,
James Expat-blog Experts Team