It sounds like COSTCO is the WALMART of wine purchasing. Like Walmart they pay very little if anything to importers/wholesalers etc. I'm sure they work with a negociant. In Maryland, in theory you still can't quote two different prices for retail outlets for one case(Volume discount is mew & being appealed), and there should only be one source for a winery for the whole State but the state ignores this for Bordeaux because of the multplicity of sources ie one house buys from Classic, another from Bobbie Kacher, it would be very labour intensive to review entire listings of chateaus if there is an listing for a winery--also if your filing quotes the label selectively, to the bureaucrat you have two different wines. So House A list Marguax 1st Grand Cru, another lists Chateau Marguax and gets away with it.
As to NC I have no idea anymore how they sell wine other than it must be through a wholesaler and they allow regional wholesalers so as an importer you're selling to Fat Cat Wholesaler in Ashville and Tres Chic Wines in Charlotte and at differing prices. Costco has to "clear" wine often through a company they secretly own or support, who charges virtually nothing for the transaction.
Another variable is the "gray market" where there is an authorized importer in a state, region or country; a gray marketer violates the exclusivity but is allowed to sell to wholesalers by most states. When I worked for a small wholesaler, a prominent store was shut out on Guigal La-Las, they arranged through a gray marketer and the company I was with to get a few cases for customers. The gray marketer dealt in Bordeaux, so we couldn't say no to the transaction, which didn't add a penny to the wholesaler's coffers.