I have not encountered a domain registar/snap company that require Canada buyers to pay GST/PST.
I went to read its (Pool.com) FAQ and found this..
How much does it cost to buy a domain at Pool.com?
It depends on the type of sale. There is no fee to place a backorder on a deleting domain on Pool.com. You pay only if you win on Pool.com. Your winning bid will be the final price. For domains purchased in the Domain Marketplace, the price paid by the buyer depends on the type of sale (Fixed Price, Auction, Dutch Auction).
Try reviewing your order email. If it is really tax, it will be stated very clearly. Otherwise, contact Pool and ask if they do charge taxes for Canada buyers. Hope this helps. *cheers*