Is it mukzah? as i recently visited an out of town shul. They gave an envelope that lists the name and address of where the check should go. There’s an eruv so they expect you to take it back to your hotel. I think it may be mukza machmas chisoron kis, or kli shemelachto lissur.
It is hard to answer definitively without hearing the other side, why they feel that it is permitted, but based solely on what you are saying, it sounds correct that the envelopes are indeed muktza. The reason would be (aside from the question if envelopes are muktza on shabbos or not) because this envelope is designated for a monetary transaction it would be at least a kli shemilachto l’issur. Although it is permitted to move a kli shemilachto l’issur, in this case the reason for taking to back to the hotel would be so that it doesn’t get lost and so it can be used after shabbos, which would not be a permitted reason to move it. Additionally, as you wrote, it may be muktza machmas chisarion kis, although the envelope doesn’t have much value, however right now you want to keep it in a safe place in order to send it to the shul. There may be another issue here, of zulusa d’shabbos, because this envelope serves as a sort of bill.
As a side point, see Shut Toras Chaim ( R’ D. Travis) 4 who talks about this very question, and he says that this practice should not be done, however he gives a very nice idea, that the shul can give the people that get an aliya a paper that talks about the importance of giving tzedakah, and on the bottom of it the shuls address!!
Shut Toras Chaim ( R’ D. Travis) 4, .Luach Muktza pg. 110, Beer Moshe 8-76 (5).