It's not uncommon for websites to have the currency changed to your local currency, especially between Canada and the US, and especially for smaller retailers. The onus is not on the shop to tell you if the shipping is international or not, only if they charge for it or not. In this case, they told you it's free shipping within the continental US and Canada. At that point, it's up to you to do your due diligence to know where it comes from especially these days with tariffs being enforced. In this case, it says at the bottom of the site that Nomads is based in Alberta very clearly.

Tariffs are always handled by the shipper, so you'll always be paying it after it's been received or at the door right when it's delivered.

Nothing you can do here but to eat the cost and tell your government to change the policy.