As long as your total AFTER use of coupons and taxes is $20 you are ok. So say you shop for $30, you can use coupons for $10 MAXIMUM. Your total would then be $20 PLUS tax, so in Alberta (where I live) it would have to be $21.00 (5%GST).
Also remember, that say you buy soda or milk, the deposit and recycling fees should be over and above the $20. So say you buy 4 litres of milk for $4.00, and the recycling fee is 0.05cents and container deposit is 0.25cents, the 0.30cents is not included in the $20.00 total.
In order to be safe, I always spend $21 or $22 - at least. In that way you are covered.