Please keep in mind that a bag's authenticity can only be guaranteed if and only if it was purchased from an authorized retailer of a particular brand (boutiques,
Nordstrom, Bergdorf Goodman, Neiman Marcus, Barney's, Bloomingdale's, Saks, Eluxury, NAP, etc). Buying designer handbags from retail partners is the safest bet.
Costco is one of my favorite stores, but it's spelled out on their site that they are not an authorized dealer of MJ bags. There is always a possibility that their buyer got the bags from the wrong source (just like fake Balenciaga bags on their site several months ago).
"This item is covered by Costco's guarantee to refund your purchase price if you are not completely satisfied. Costco's guarantee applies, even though this item may not be covered by the manufacturer's warranty, because Costco is not an "authorized" dealer of the merchandise."