We have a similar system here in Canada, a committee that looks at stamp proposals. As a graphic designer and illustrator, I've submitted my portfolio to them, and received a very polite 'don't call us, we'll call you' letter in response. It's a pretty exclusive club. Canada Post
did hold a stamp design contest a few years ago for their annual Mental Health stamp. It was a vote by the public kind of affair, and I submitted two designs. One of my designs rated about 43 out of over 300 submissions, but obviously I wasn't able to quit my day job. Still a dream of mine to design a stamp, but it'll probably always remain just that.