A letter, so you gave them your address ! Oh dear.
I gave up most charities that I contributed to via the Royal Mail when I changed address. I found most spread the word around that I was a sucker giving out money, and I was inundated. Worse, because there were so many, each got less than they otherwise would, and they wasted that by sending more begging letters and guilt gifts throughout the year. I doubt much went on the thing it was given for.
Now have a select few, and even they send "emergency appeals" for my recycling bin. IMO best to simply put something in the tin, anonymously, when you see one you approve of.