The seller cannot charge more than the advertised shipping and handling price to the buyer's destination.
But if there was no advertised S&H to Canada, the seller can charge whatever seems fair.
And the buyer can negotiate that price before bidding.
So if there was no shipping fee given for Canada, yes, the seller is not required to give you the advertised fee for an entirely different country.
Normally, Best Offers do not affect the shipping fee which in theory reflects only the seller's actual costs for shipping. Thus, if an item is offered at $100 + $50 shipping, a Best Offer of $50 would be deemed to be $50+$50 shipping for a total of $100.