If you can use it, here is an English usage worksheet on understanding the difference between stationary and stationery and their use. The second, with the e, is the stuff you use to prepare correspondence and birthday cards, i.e. paper, envelopes, and cards.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.