If you're booking on the Eurostar UK site, you'll have to pay a £4 credit card fee. Since you're in Canada, you'll probably be paying an overseas usage fee on your credit card too (eg 3%)
Easiest thing is to book on the Eurostar Belgian site, in English. Charged in Euros, but either GBP or EUR are overseas for you. The Eurostar Belgian site has no credit card fees
Otherwise, book with
Loco2 (2.5% fee for using a credit card, none for debit card, bills in GBP) or
Captain Train (no credit card fee, bills in EUR) - both of those sites sell Eurostar tickets for the same price as Eurostar themselves, but with better websites!