For the most part, yes, you should usually go through a celebrity’s representative to make arrangements. It shows that you are legitimate and are following the correct paths for contacting the celebrity. It is also the only route for securing a signed agreement that your celebrity partner will commit to the charitable tasks you’ve asked about.
If you are asking a celebrity to make an appearance at your event, then you always, always contact their representative first.
However, the exception to going through a representative may be if you are simply asking for the celeb to donate signed items. You could still contact the rep first in this situation, and it may in fact be the best way to go. But if you are a small organization or a group doing a one-off fundraiser, it’s possible to simply send an item through fan mail to be signed. The upside to this option is it’s easier. All you have to do is find the correct fan mail address and send off your item with a short note explaining you’d like an autograph. The downside is that you have no guarantee of getting the autograph or getting it before your event.