Answer:
stationery
Step-by-step explanation:
Yes, it's easy to confuse those two, one letter can make a huge difference.
stationery : is for paper, envelopes and all similar paper office supplies. I use the "E" at the end of the word to identify "envelope" in my head to be able to distinguish them more easily.
stationary: That's to describe something that isn't moving, that is fixed.