If you are buying your shipping labels on eBay, where you get a discount, the cost of the label is taken from your customer's payment immediately.
If you are buying your labels (or postage) at the PO counter, you will pay full price with your own money.
If you are buying your labels at the PO counter, get a Solutions for Small Business account from Canada Post for their discount.s
https://sso-osu.canadapost-postescanada.ca/pfe-pap/en/registration/business?sourceApp=SMB&sourceUrl=...
You can also buy mint discount postage here on eBay. Avoid the booklets of "P" stamps, which are very likely to be Chinese counterfeits but packages of assorted older stamps are safe. Some large businesses like CostCo also sell current booklets at a discount.