Walking around the streets in New York, I do see a lot of bodegas (like a corner deli elsewhere but carrying a lot more interesting stuff!) and such that say “Stationary” on the awning. And I also find that error a lot in manuscripts and books! So my little personal trick with this is to ask if they will need an Envelope. If so, then use the “e”. If not, the message isn’t going anywhere and the stationary version is right.
I’ll see if I can find a picture for this--it’s usually not too hard, but I don't always track them. :)