I asked AI for a more complete explanation so please correct this if wrong... One went into a postal outlet and obtained (purchased?) a blank money order then bought and attached sufficient scrip to pay for some small expense. The money order with scrip affixed was then mailed to the recipient who took it to their local postal outlet for reimbursement.

Image from An Introduction to Canadian Revenue Stamps; Postal Note and Scrip Stamps by Christopher D. Ryan, BNAPS.