I have only been to a grocery store two times, and the target liquor section two times (lol), over the past month, but at least at the stores I have gone to, they've had a staff person dedicated near the door / carts, cleaning them prior to anyone grabbing one. People seem to follow the six foot rule, since most people by now are kind of paranoid anyway, I suppose. At my recent trip to the grocery store, almost everyone had a covering/mask. Target was a little more loose, with more maskless people.
I used to shop at trader Joe's before all of this, but because it is smaller, and because they seem to be more militant like your Costco example, I haven't bothered going in because there's always been a line out the door and around the back of the building of people waiting to get in, and it's not worth it for me. I'm bummed. When I tell people this they are surprised, but my grocery costs are at least 1/3 cheaper at trader Joe's than any larger store, so I'm bummed that I'm now paying noticeably more for groceries.
I admit early on I kinda had an eyeroll reaction to mandates or strong suggestions on wearing a face covering, since I knew anything non medical grade wasn't going to be super effective, but anything is better than nothing I suppose, and it is nice to know that everyone's coughs and sneezes, at least, will be behind masks. I am eagerly awaiting two I ordered, which won't arrive for another week at least, as my bandana thing is not practical at all. Too, when I return to work masks will be required, and I imagine they'll be required for a long time to come, if not a new permanent policy.
Edit: I do think it's a store by store thing, though, and in some cases a company thing. Since I never shop at Walmart I wouldn't know how it in general is handling things, but a day or so ago I saw a news item about one store in the area being shut down by the state due to three deaths with ties to it, and many infections. So it seems the public health dept of the state is keeping tabs on these things.