For abbreviations - both acronyms and initialisms - where the last word in the abbreviation is the noun, I'm accustomed to adding an 's' with no apostrophe (e.g. ATMs), as described in the answer to this question: What is the correct way to pluralize an acronym?
But how do you pluralize abbreviations where the noun is not the last word? E.g. 'Point of Presence' (POP)
I can see how 'POPs' could read 'Point of Presences', when the intention is 'Points of Presence.'