May one open an envelope on Shabbos when it is very likely the person opening the envelope will keep the it to hold it’s contents in indefinitely (such as invitation that has numerous inserts)?
The envelope may not be opened in a way that it can be reused. Even ripping it open is not permitted according to numerous poskim because te information is not important to know for shabbos.
Igros Moshe O:CH 5-25(5), Ashrei Hoish O:CH 16-13, R’ Sheinberg zt”l Chidushei Basra on M:B 307-53, Ayil Meshukash 6 ftnt. 22 in the name of R’ N. Karelitz shlit”a, Shmiras Shabbos Khilchoso 28-6 29 ftnt. 117.