One letter apart, entirely different meanings, and no way to tell them apart by sound. This is a pure spelling problem rather than a usage one, which makes it a good candidate for a memory device — and there happens to be an unusually clean one. Both words come from the same root, oddly: medieval booksellers had fixed stalls rather than travelling, so a "stationer" was a seller who stayed put.
An adjective meaning not moving, fixed in place.
A noun meaning writing materials — paper, envelopes, pens.
Quick way to remember
Stationery contains an "e," and so does envelope — paper things. Stationary contains an "a," and so does "stay" — standing still. If you can picture it in a cupboard, it takes the "e."