Post Office note about label showing cost of carriage (7)
I believe the answer is:
portage
'cost of carriage' is the definition.
(portage can mean the cost of transporting cargo)
'post office note about label' is the wordplay.
'post office' becomes 'po' (abbreviation).
'note' becomes 're' (as in do-re-mi).
'about' indicates putting letters inside.
'label' becomes 'tag' (tag is a kind of label).
're' placed around 'tag' is 'rtage'.
'po'+'rtage'='PORTAGE'
'showing' acts as a link.
(Other definitions for portage that I've seen before include "Carrying boat overland" , "Carriage" , "Cost of carrying or transporting" , "Transport between waterways" .)