I suspect this is because you are bidding on items that are in the USA and what you are seeing is the shipping cost for US customers.
However, since you think this is happening consistently, your best bet is to stick with your original bid.
Bid once, bid your max. Many successful buyers add "bid late".
With the recent introduction of the Global Shipping Program, be very careful when you are bidding over $20 for an item. At that point, Canada starts charging sales tax and sometimes duty on your imports.
If the seller is using USPS/Canada Post delivery, that means you can plan to pay $10 service charge and whatever your provincial sales tax is. There is rarely any duty to pay, and it is very variable, depending on what the item is and where it was manufactured.
And Canada Post is notorious for letting many parcels slide.
BUT if the seller is using the GSP, you will be charged all applicable duty, tax and fees before the item is shipped to you. And you definitely will be charged. No slippage.