Most British Forces Post Offices (or BFPOs for short) have issued postal orders. BFPOs are located in British military bases around the world. Postal Orders issued at BFPOs enable members of the armed forces to purchase items by mail order and to send money home.
Collecting
Postal Orders issued by BFPOs are sought after by collectors because many BFPOs are only open for a short time and access to them may be restricted to military personnel.
References
British Forces Post Office postal orders Wikipedia(Text) CC BY-SA