To my knowledge, Costco accepts any of the GFSI benchmarked schemes, with some restrictions on all of them. They also have their own additional requirements to all of the schemes.
The best answer, however, to be safe, is ask Costco. They are really the only ones who can speak for what they will or will not accept. The same would be for pretty much any potential new customer, don't guess, don't go on opinion, go to the source and ask - it's in their best interest for you to have good information.
Edited by john.kukoly, 14 February 2014 - 02:46 PM.