I guess it's worth pointing out that employees of Costco are human, too. Just because the corporation has a policy doesn't mean every employee agrees with it. They are instructed to follow certain rules, and most do. It sounds like the receipt checker was probably "done" with things, too. Can't say as I blame them, since they're on the front lines with shoppers who think the rules don't apply to them, and since they're at the door, they get the brunt of that angry member's attitude. My spouse works at the Refund Desk and regularly gets yelled at because some item is out of stock, or because someone had trouble at the gas station or because it took too long to get their lunch at the Food Court. Some Costco shoppers are just entitled a$$holes.
But my greater question is if it was such a huge deal for you, did you ask them about the carts? Is it possible they were sanitizing them in the cart corrals out in the parking lot, so they didn't have to take the time when bringing them back to the building? They may have already been sanitized before being offered to shoppers.
And if you did ask, and they said they weren't sanitizing carts any longer, did you ask them why not? I'd like to know the reason, since last I'd heard, the Corporate policy is to sanitize them.
I'm sorry you couldn't find the one item you went in to purchase. I think if I was only after one thing, given the mask thing and all, I'd have just gone to a grocery store. But that's me.
Dave