I think using no form of address is usually the safest option in a shop nowadays, Phoenix, though I don't dislike 'Madam' (or Sir) to such an extent that I would even think of complaining if someone addressed me that way. I just find it outdated.
The problem is that IMO it no longer seems appropriate for a young woman.
I doubt if any us would be likely to use it to address someone that we guessed was in their twenties, or even thirties, would we? Well I certainly wouldn't, anyway.
This means that the speaker has to make a decision based on how old someone looks to them, which can open up a whole can of worms. ?
How much do you spend on yourself?
Bought the wrong Hot Cross Buns 😩
Water Pollution -“ A National Disgrace”? A case for renationalisation?