Getting things to your customers costs money, and that cost comes off your business income. On Form T2125, it goes on line 9275, Delivery, freight, and express.
What belongs here
The cost incurred in the year of delivery, freight, and express that relates to your business. In practice that is a courier bill or a shipping invoice — a tracked parcel sent to a customer, a pallet moved by a freight carrier.
What does not belong here
Freight on goods you bought to resell or to manufacture. Delivery, freight, and express charges on incoming purchases join those purchases in cost of goods sold, in Part 3 of the form, rather than sitting here among your expenses.
Ordinary postage. Stamps and letter mail are office expenses and go on line 8810. It is a courier-grade shipment, the kind that comes with a tracking number, that belongs on this line.
What to keep
Keep a record of all your income and expenses, with the documents behind them. A courier or freight invoice showing the carrier, the date, and what was shipped is the document for this line.
Keep the invoice that shows the direction of the shipment where you can. What was going out to a customer belongs here; what was coming in with your purchases does not.
Don't send them in with your return. Keep them in case the Canada Revenue Agency (CRA) asks to see them. Business records follow the same six-year retention rule as your personal documents.
Official CRA sources
Source: Canada Revenue Agency (canada.ca). Not tax advice.