Stationery and envelopes are literally the same thing, but it's a pun on the word "stationary", which means "to be still."
Here's a more formal explanation from google:
“To push the envelope” is an expression that means to exceed or try to exceed the normal limits. “Stationery” is writing materials such as paper and envelopes. “Stationary” (-ary) means to not move. This pun plays with the word stationery/ stationery and the idea of pushing something (moving something) and staying still (stationary).
I hope this helps! Good luck!
