You don't unless you need a "victim' badge for your Girl Guide sash.
If the seller did not charge you 'enough' to cover his costs, that's his problem. He cannot charge you more than he advertised.
One note: if you somehow paid the cost for US domestic shipping and you are in Canada, the seller is entitled to cancel your short payment and re-invoice you.
But if he said it would be $X to ship to Canada , he can't change it to $X+Y.
