Dios Mio! What did we see at the original urban wholesale-type big box in San Francisco but Coca Cola straight outta Mexicali. The reason why Mexicoke has a cult following here in the U.S. is the use of old-school sugar from cane, as opposed to the sweetener used by American bottlers these days: corn syrup. Some say that Coke is Coke, so whichever sweetener used doesn't matter. Others strongly disagree - they feel that the Coke hecho'ed en Mexico tastes like what Americans drank in the long long ago, back before 1985's New Coke/Coca Cola II Project Kansas debacle, back when sugar was the sweetener used by all U.S. bottlers.
Costco has conformed to CA and U.S. rules, such as CRV (the sort-of deposit you pay for the bottle) and "nutrition" labeling, so everything appears to be nice and legal. Of course you could always get your sugar water fix at some smaller grocers or taquerias by buying surprisingly expensive "bootlegged" bottles one at a time, but Costco will let Cokeheads stock up by the case at a relatively low price. You can read what Coke corporate HQ in Hotlanta, GA thinks of the importation of Coke from south of the border in an excellent article h'yuh. So, check it out. Or not. Tome whatever you want, we don't care.