The phrase "to open the envelope" means to reveal or discover something that was previously unknown or kept secret.
Sentences with «to open the envelope»
When my results came back, I ripped open the envelope to see what they said. (nakedpastor.com)
You may have learned to recognize pre-approved credit offers without opening the envelope and toss them directly into the recycle bin. (mymoneycounselor.com)
The checking of the returns shall be done before opening any envelope. (bloodhoundswest.org)