The variables would include whether or not you are bidding on an auction and what province you live in.
In an auction the price is not determined until the last bid. Then duty (if any) and taxes are based on that. In addition, Pitney Bowes may charge more than their basic $4.95 service fee for more expensive items.
And provinicial sales tax varies from 5% to 15% and the program can't show that until it knows what province you live in. (I could be wrong about this one, since eBay can tell you what shipping will be when you open the listing and before bidding because your postal code is attached to your ID.)
