The A, of course, stands for 'avoid thinking of envelopes'.

Like everyone else, I used to confuse the words 'stationery', meaning materials to do with writing; and 'stationary', meaning not in motion. And like many people, I now tell them apart with the mnemonic that the E in 'stationery' stands for 'envelopes'. 

But it only today occurred to me to wonder why two words with such different meanings should be so similar. So, I looked it up... and it turns out that the first stationers, in the Middle Ages, were scribes and paper merchants given licences, typically by universities or law courts, to ply their trade from permanent booths- or stations- as opposed to their competitors, who were itinerant peddlars.

So, pleasingly: stationers sell stationery because their stations were stationary. 

I thought you'd like to know.