I sell to Canada all the time (CAnn..you have a scale..why do you have to go to the PO to get the cost? If you need help, email me, and I'll walk you through it..it's as easy as getting domestic costs). But I state my shipping for most items, and on items where I don't, I tell them to e-mail. Since the $2.50 price was to US, I feel buyers have an obligation to email and ask what the shipping price is to Canada.
The seller may pay someone to do their packing, and since there is a form to fill out, and we can't use priority boxes, (or they may have to make a trip to the PO instead of having their postman take the package), that price may not be unreasonable. But even if you think it is, they don't deserve a neg or even a nuetral because of that, since you didn't ask. Now, do they deserve a neg/neutral because you paid extra customs fees, since they put the incorrect amount...yes. If it really bothers you, I would do two things..I would write and ask why the high shipping..and they may tell you..and if you had to pay extra customs, I would state that and ask for that difference back, because of their mistake), or in my posi (if it didn't cost you extra) I would just state, shipping seemed high, wrong amount put on customs form.
Just so you know, in eBay's terrible feedback system, they very well may leave you a neg in retaliation..it's the price you may have to pay, if you leave a neutral/neg. Unfair? Yes, but that's the way it is.