Sam's Club offers key item Costco refuses to sell
Both warehouse clubs are similar but they're not identical in what they offer members.

When you join Costco or Sam's Club, most people consider the membership fee relatively irrelevant. If you buy regularly at either chain, you certainly get that money back in savings over time and one big-ticket purchase can more than fully pay back the cost of joining.
Costco COST sells two memberships. A Gold Star membership costs $65 and gets you access to the company's warehouses, gas stations, and its website.
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Nearly half of members, however, opt for an Executive Membership which costs twice as much ($130) but comes with 2% cash back up to $1,250 annually. At the end of the second quarter, Costco (COST) had 36.9 million paid Executive Memberships, up 9.1% versus last year.
"Executive Members now represent 47.1% of paid members and 73.8% of worldwide sales," according to CFO Gary Millerchip.
Sam's Club offers a similar structure. A Club Membership costs $50 per year and gets you access to the chain's website, warehouse clubs, and gas stations.