Costco is only cheaper if (a) you will always use the full amount of product before it spoils or you no longer need it and (b) the cost of traveling the extra d

Costco is only cheaper if (a) you will always use the full amount of product before it spoils or you no longer need it and (b) the cost of traveling the extra distance and spending more time shopping is lower than what you save on the purchase itself. It's not the case that these are both satisfied for everyone.