I buy mostly food at costco, but it has EVERYTHING. Jewelry, tires, frozen desserts, office supplies, fresh flowers, prescriptions, eye glasses, computers...everything. Occasionally, they also have designer clothing at a fantastic price. I'm sure you can get a pass to just look around if you're considering membership. I've never been to a Sam's Club, but Costco has so many "extras" like ordering custom cakes and catering platters, an on site pharmacy, even eye exams before you get your glasses, and fantastic seasonal sections that it seems to be a great value.
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