I recently joined Pixel Shavings, a small group of children's illustrators and writers, and we're about to do out a postcard mailing. I've never done a mailing before so I'm excited and curious about the process.
As I started doing some research, I came across a wonderful post by Victoria Jamieson in the #kidlitart archives about what made her immediately discard artist postcards when she worked for children's book publishers. Her reasons for immediate rejection were basically:
1) too editorial
2) too mass market
3) too sci-fi
4) too weird
5) not telling a narrative story
Read Victoria's full post here: Get 'er done.