The people who run charities are not generally stupid. It therefore seems probable that they get a return on those follow-up mailings that exceeds their cost.

But yeah. I donated to NPR once thinking it would make me feel less guilty during the pledge drives. It did..sort of. It turned the guilt to rage as my subconscious started screaming I already donated! Shut up, already! And then the letters. It just wasn't worth the annoyance.