You do before purchase, but generally not as part of the storefront product listing. That necessitates the seller already knowing where you'll be shipping it to

You do before purchase, but generally not as part of the storefront product listing. That necessitates the seller already knowing where you'll be shipping it to, and that's traditionally the last step before purchase (whether that's best or not I can't say). Sales tax is also usually added in at the very end, again because it depends on where the buyer is. The difference here is that the "hidden junk fees" are just extra money they charge, with no real defence. For example, Ticketmaster loves their arbitrary "convenience fee", Airbnb was long known for "cleaning fees" (though they've improved here), etc. These are things that the seller knows they're going to charge beforehand and clearly they do this purely to trick our brains into thinking things are cheaper.