Work up on coach carriage (7)
I believe the answer is:
Here is my best explanation:
'carriage' is the definition.
'work up on coach' is the subsidiary indication.
'work' becomes 'op' (common abbreviation for 'opus').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'on' is a charade indicator (letters next to each other).
'coach' becomes 'stage' (stage is a kind of coach**).
'op' written backwards gives 'po'.
(Other definitions for postage that I've seen before include "Charge for 23 Acrosses", "Amount paid to mail", "cost of delivery", "Money charged for delivery", "Cost of sending letter", "Mailing cost", "Cost to send a mail item", "It could be first-class", "Delivery cost" and "Cost of corresponding".)