Since Costco has an exclusive deal with CIBC+Mastercard, and Loblaws/Superstore/NoFrills has the PC financial Mastercard, I think it's safe to say these places won't charge the extra fee as it would directly conflict with their exclusive cards. Online merchants like Amazon won't charge it either because they can't accept cash. It then stands that other megacorporations like Walmart also won't charge it because they have to compete with Amazon/Costco.

I think this ruling will likely only apply to businesses that have us by the balls, like telecommunications and utilities providers. Anyone else who tries charging extra will get wrecked by people flocking to their competitors.