Blank envelopes are never a good idea, they create an instant defensive response mentally. Even a business likes the personal approach, it's human beings picking them up.
A little more preparation and care might just double you're strike rate next time. Spend some time amassing lists of the sort of places you might approach, they might even be available online, I don't know.
Address the envelopes to the boss's title, be sure to get it right, another positive ego-trigger.
Make the envelopes appear business-like, it opens portals in their minds, fits their pre-conceptions, and means mentally they are taking you more seriously.
Be sure to keep it relatively light, whatever the cause, you need to keep them up-beat, in a positive frame of mind.
Finish on a high, a happy note, it makes a big difference to their emotional responses.
I've done a bit of fund-raising in my time, successfully, and researched it before-hand.
Hope that helps.
Thanks, I was basically just asked to put the letters into envelopes (after I offered to type up the letter as a favour). I personally don't have the time to do yet another job that isn't mine, when I'm trying to do quite a lot of my own jobs plus a few others anyway.
As I said in my OP, I know it would be better to have addressed them to companys, but the people in charge just asked that the letters be taken around and given to buisnesses, there was no guidance as to where they wanted them to go.