What happens when you go over on a coupon with a weight limit?

I have a coupon for salmon, limit 2lbs. if I go to the fish counter and ask them to give me 2lbs, they could very well throw me a bag with 2.1lbs. Does this mean I get the discount for 2lbs, and the extra 0.1 is full price? Or does the whole thing get kicked off the coupon and then I end up paying full price? These are the things that keep me up at night. Thanks for coming to my 300 character Ted talk.