I donate regularly to a few non-profits that focus on issues that matter to me. These non-profits tend to bombard me with postal mail and sometimes phone solicitations. I do not terribly mind this practice because after all sending reminders is one way to stay relevant in the mind of a donor. But the problem is that many other non-profits who possibly share marketing databases with the non-profits you donate to, also tend to send a lot of junk mail your way.
What happens is that if you make a one-time donation to a non-profit, a sizable percentage of your donation will be spent on sending you postal mail. And you will be doing more harm than good if that donation is a one-time small donation. The affiliate postal mails that will also be sent to you will pretty much negate that donation you made, in the wasted printing and mailing costs.
So its important to only donate to a few non-profits whom you plan on giving to on a long term basis and to also request the non-profits to whom you are donating to, to not share your information with affiliates of any sort.