If the pricing difference between costco and local heimish store is such a hardship on shimon, for the occasional purchase when his friend is over for a coffee, I would say it definitely a shas hadchak. in fact, i would ask reuven to bring his own milk.
Now, you addressed the milk, you did not address “dairy products”. Besides milk which costco sells as a loss leader, what other c’y dairy products cost 2.5 times their non c’y equivalents.