are there laws or traditional “best practices” about what that means? E.g. if price has changed after the order has been placed, what is supposed to happen?
Prior to Internet buying/selling, the time lag between a production run of a catalog and the time you order from it might affect a product’s price/availability. The catalog might say $10.95, but three months later when you finally receive the catalog and order from it, the actual price is now $11.95.
With Internet ordering and JIT (just in time) supply, this should not happen. However, a supplier might have a web site, but everything else in their operation is still a few centuries behind. Depending upon consumer protection laws and the wording on the web site, once you order something, you bought it at that price (offer, acceptance, consideration, done!). If the supplier’s cost jumped they have to eat the loss.
A quality seller will honor a commitment, even with a price/availability disclaimer. Also, a seller is still subject to the law, regardless of what they say their policies may be. However, many buyers fail to assert their rights (if needed) and take what’s given.