Wait for enough confirmations where the payment becomes unlikely to reverse, which of course takes time and that's the more practical blocker for regular shops to accept Bitcoin.
They aren't doing that, in places I've seen, they just take your payment and let you go away with that. (I've never paid myself, but I've seen other customers do so)
If it's raw bitcoin, they couldn't even be sure that the transaction is a valid one (that the wallet has the funds in the first place, not even talking about double spending). I suspect they use some kind of third party like Coinbase, and that there aren't really using bitcoin at all (and just use Coinbase as a bank) but I'm not sure.
Thanks. I'm not sure I understood the answer given in the specific post you're liking to (or at least, I don't see how it answers my question). A following response linked to this story: https://www.ccn.com/bitcoin-atm-double-spenders-police-need-... which gives a pretty good answer.
There are 2 approaches to Bitcoin Network usage in play. Bitcoin Core team went with RBF(Replace-By-Fee) which gives rise to the double spending problem as the funds can be re-directed before a transaction gets in to a block. And Bitcoin Cash protocol implementation removed the RBF to support trust in 0-confirmation. There are even more optimizations in BCH implementation of Bitcoin, you can review it here https://cash.coin.dance/development