Question

no matter how much you push the envelope, it will still be stationary. can you tell me the pun and the 2 meanings of it​

Answer 1

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!