obligatory stamp
stamps that must be affixed to all mail at certain times for various reasons (support after disasters, as a form of war tax, etc.); usually only on domestic letters, etc.
en-GB/əˈblɪɡətɹɪ ˈstæmp/
stamps that must be affixed to all mail at certain times for various reasons (support after disasters, as a form of war tax, etc.); usually only on domestic letters, etc.