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When someone tries to sound overly fancy with their vocabulary, and does not want to use the proper word, "suffix", they may use this term d'art. In this case, the person is creating a variation to "suffix" by using the prefix "post-" (as in "post-war" for "after the war", or "post malone" for "after the era of Karl Malone"
"As the third generation with his name, Greg G wanted to add a suffix such as 'the third/III' to his name. To try to impress the judge, he suggested that a 'postfix' should be added to his name, much to the chagrin of the courtroom"
An acknowledgment of a spoiler very much like a spoiler alert except that it's provided after rather than before the spoiler in question. See, for example, the Dinosaur Comics strip for April 15, 2011.
By extension, any would-be warning preceded by the thing it's supposed to warn against.
"So then Snape kills Dumbledore. That's a particularly infamous spoiler." "Nice postfix spoiler alert, you numskull." / A week after the drop deadline, the registrar told me I was taking the wrong classes and wouldn't be able to graduate till next semester. Talk about a postfix spoiler alert! Does he think I'm Merlin or something?