Many people think that Costco is the greatest thing since sliced bread. I've had enough experience with them to know that shopping there isn't for me. My reasons are several.
1. I have a philosophical problem with paying admission to get into a retail store. You may not agree, but I feel that an annual membership fee is a payment of admission ("We charge this fee to keep prices low").
2. I hate waiting in line behind 20 other people for the privilege of checking out, and when I get there, they won't provide me with any bags for my purchases ("That's how we keep prices low") and they won't take a check.
3. Often their goods are sold in large units, but it isn't often that I need a 55 gallon barrel of Thousand Island dressing. Sometimes when you are buying for a single household and you buy in quantity, some of the product may go bad before it's used up. That isn't thrifty. Another point to make here is that I'm not used to pricing large quantity grocery items, so I have no way of knowing if I'm getting a deal or not.
4. You have to be a careful shopper at Costco; often their pricing is higher on certain items.
5. Another way you have to be a careful shopper at Costco is to avoid the "I'm getting a special deal here" and over-buy just because you think you are getting a special deal. You may be getting deals, but you also may be buying things that aren't necessary. It's easy to leave Costco with a very long receipt, which makes you wonder, "Did I really spend that much?"
Now, I've got a real problem with buying gasoline from Costco. I've driven by one of the Costco stores near me several times. In the area where they sell gasoline, the cars are lined up (no exaggeration) 10 and 12 deep to get up to the pumps. At present, their unleaded gasoline is about 15 to 20 cents below other retailers, depending upon whether you buy at Arco or Union 76. Let me tell, you, all those loyal Costco customers waiting in line behind 10 cars to save 15 cent a gallon have got to be nuts. No way am I going to wait in line behind all those jabbering, disorganized, pushy (never mind) to save 15 cents a gallon. Think about it; you have to wait for every driver of every car to get out, pay, pump their gas, fiddle-fart around with their seat belts, kids in car, put receipts away, etc, times however many cars are ahead of you. I've got better things to do with my time than that. That isn't saving money, it's wasting time.
I'll leave you with this final thought. If you could drive right up to the pump in some kind of express lane for special people and avoid waiting behind 10 cars ahead of you, wouldn't you be willing to pay 2 or 3 dollars just for that privilege? That's basically what you are doing by going straight on by Costco and going to Chevron (or similar) to buy your gasoline. I'm happy to pay a couple of dollars extra to avoid that jabbering bedlam in the gas pump line at Costco.
If you are looking for bargains in grocery items, you can often save more money at places like Grocery Outlet or Save-a-Lot, or similar discount stores in your area.