This concept has been studied already extensively, e.g [1] (in 2000!) by people like Rivest and Chaum, who have actual decade-old competence in that field.
[1] https://people.csail.mit.edu/rivest/pubs/pubs/LRSW99.pdf
https://www.nytimes.com/2006/02/05/technology/postage-is-due...