They don't have to accept if you pay in pennies. from Wiki:
Legal tender of Canadian coinage is governed by the Currency Act which sets out limits of:- 40 dollars if the denomination is 2 dollars or greater but does not exceed 10 dollars;
- 25 dollars if the denomination is 1 dollar;
- 10 dollars if the denomination is 10 cents or greater, but less than 1 dollar;
- 5 dollars if the denomination is 5 cents;
- 25 cents if the denomination is 1 cent.
I remember this from Grade 12 Economics.