Found this thread while searching for correos Mexico. I recently have had a similar experience looking for stamps to buy directly from Mexico's official philatelic website, "
http://www.tienda.sepomex.gob.mx"I've tried in several browsers (Chrome, Edge, Safari) and they all alert me the website is "Not Secure". To the best of my basic understanding, this is due to the poor encryption used on the website which runs the risk of having personal data intercepted. Combine this warning with the anecdotes of scammers bribing officials for foreigner's information, I'm extremely hesitant to register for an account on that website.
It's a shame since Mexico has many great stamps and the website has a large collection available. I hope one day to find a similar source for modern Mexico stamps.