Hi Maria,
Sadly it's not, under the present setup.
Well, let me rephrase that. Anything is
possible in code, but the cost of introducing conditional exceptions to a simple algorithm (like "are these two countries the same or not?") is high, because it has to break the flow of processing to check for the exceptional condition, and it has to check for the exception in EVERY shipping calculation!

This not only increases processing time across the board, but it also makes the code harder to maintain and update because the logic is more complex and therefore the effect of changing it - or the other functions it has to interact with - much less predictable.
I do understand the problem, and the only real solution to it is to create shipping "zones" that the rates are based on. Then multiple countries (like the US and Canada) can be put in the same zone, or a single country can be broken into multiple zones, or whatever. This gives you unlimited flexibility in assigning shipping rates to locations, and I do plan to introduce a zone system at some point, hopefully in the near future (the database is already set up to accommodate it.) But that has its own knotty problems, in that it puts the burden on each booth owner to come up with a whole
schedule of shipping rates for each product instead of the two that are required now, and that's a lot to ask ... plus, the zones have to be defined in such a way that the zone a buyer is in (relative to each seller) can be determined from the shipping address!
Now having been through all that

and in the meantime ... the case for an exception between the US and Canada could be strong enough to make it worth doing anyway, since such a high percentage of our members and visitors are from these two countries ... but only if there is general agereement among all our booth owners from US and Canada that they should consider each other "domestic" for shipping purposes. If it has to be done selectively for different booths and products then it really will have to wait for the zone system, 'cause that's just too much spaghetti code to be smart programming practice!
If you want to take a poll on this, I'll go along with it. I imagine it will
probably the case that most folks will go for it ... although for folks using shipping methods other than the postal system there still are significant cost differences!
Shipping rates are the problem child of e-commerce, no doubt about it!

Anyway let me know what the consensus is.
Love and blithers,

Greg