Back when DP was acting as a church secretary, one of the first duties she was given, and stressed to make sure she understood it, was to send thank you notes for every donation, within a week of the receipt of the donation. She was told to send them out even for things like cookies for coffee hour and flowers for the altar. Sure there were a few folks that ended up with 'bulk' thank yous (the same families would bring in cookies, flowers, donate to the rummage sale, add an extra $$ to the minister's emergency fund, and bring in fertilizer for the plants in the gardens, and they would get one long thank you). But when they started tracking those things, they found that donations were up a good percentage, and repeat donations up a really large percentage, from the year before. When asked, they often cited the thank you notes! So it's not just good manners, but good business sense to do those things when you're a charity.