Is it legal to eat packaged food while in a supermarket and pay for it when you get to the counter?

It depends on the super market managment. eating of food is legal but in the super market frist you pay and then you eat in the specific area. 

Hi! From fresh produce to electronics, Costco ensures its products meet or exceed customer expectations. Commitment to quality leads to customer satisfaction and strengthens their trust in the brand. And if for some reason the buyer is dissatisfied with his product, then you don’t have to look for a costco phone number but contact a customer service representative online, and he will certainly try to resolve this unpleasant situation since customer satisfaction is always in the first place for them.