I know a few people who work at Costco and it is probably the highest paying in the retail store industry. Experienced clerks can make as much as $20/hr. with full union benefits.
Costco does NOT stand for China Overseas Tranportation Co. They are based out of Issaquah, WA (~17 miles east of Seattle) and began doing business in the Pacific NW some 25 years ago. They merged with San Diego based Price Club in the early 90s.
Personally though, I don't much like Costco. Their stores are always crowded with long lines at the registers, their parking lots are nightmares, they don't advertise their selection and prices (so I don't know what available beforehand), and their actual prices are only good deals if buy more in bulk that you probably actually need. It's just too hit-and-miss for me. And a gallon of pickles for $3 bucks isn't actually a good deal if the pickles rot before you're halfway through the jar. Bulk isn't always a good deal -- sometimes you're being tricked into over-consumerism.
Still, my GF keeps a membership there (which means I have an automatic membership, Costo is cool that way), and hey I just picked up a year's supply of razors for 15 bucks last week. Can't knock that. They have good stuff, it's just hit-and-miss. And because they don't advertise much, I can easily walk into one of the stores and walk out without finding any deals.