It's an older kids movie. I remember that there were a couple of children. And some girl had a power to become small and to go in the stamp (stamp on the letter). Doing so, she travelled in order to maybe find something? There was some bad man who collected stamps, and her friends were trying not to allow him to get a stamp with their friend in it, because it would be a disaster. I think that in the end she end up in some tribe of black people, and they were singing... I don't know, sth like that. Quinn F, thanks in advance :) ;)
