ZT systems is another "custom" or 3rd party builder of pre-build PCs. They're making typical PCs that anyone can make they simply put them together and order "models" like like any other vendor. They are more open to customer needs though I'm sure the ones sold at Costco are XYZmodel1 then a minor difference to make XYZmodel2, etc.. Costco also tends to offer what they believe buyers will want and may drop them and go with another vendor that's flexible for such orders/pre-builds.
As for customer service, that's unknown to me as they don't come across my repair desk. That maybe a good thing, but they don't sell a zillion systems, either. Check the BBB for any negative reports or any of consumer complaint websites as well.
http://www.ztsystems.com/
Compare the ZT website with whatever Costco is offering.
tada -----Willy 
My PC has suddenly, and unexpectedly, died (it is either the motherboard, or RAM, or something else, but its gone). So I'm in the market for a replacement. I've been checking out what both Costco and Sam's Club have to offer. (I don't have something like Fry's available to me, so that's out.) Both Sam's Club and Costco sell Dell's and HP's, but Costco also sells some CPU boxes only by a company called ZT Systems. I've never heard of them, even if they've been around for 15 years. Would someone please tell me what they're like, how good their machines are, their customer service, etc, please?
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