In Canada, the Currency Act, RSC 1985, c C-52, s 8, defines how payments may be legally made. (Among other things, payments in coins are restricted so as not to create a burden, as happened long ago when someone in the US deliberately sought to pay hundreds of dollars' worth of parking tickets in pennies.)
In practice, yes, it may be difficult in the short term to compel a store to accept a legal payment in Canadian currency. But refusal of the payment is illegal.