Which cosmetics are the most advantageous to buy when taking advantage of the duty-free opportunity?

I heard that these days, duty-free shopping isn't as advantageous as it used to be.

Still, there were a lot of products that were cheaper than buying them in the store.

There are many duty-free shops, and the sale details vary at each, so I'm not sure what the best way to shop is.

Also, are there any cosmetic products or brands that I absolutely must buy at duty-free shops?

I would appreciate it if you could give me a recommendation!