Which is cheaper, online duty-free shops or airport duty-free shops?

In the past, whenever I traveled, I always ended up buying cosmetics at the airport duty-free shop because I couldn't prepare in advance. Is it cheaper to buy from online duty-free shops? My friend always buys things in advance from online duty-free stores. When I asked if she did it because it was cheaper, she said she didn't know and it was just a habit, so I just accepted it. But now I'm curious. I'm thinking of buying some cosmetics for my vacation, and I'm wondering if there is even the slightest price advantage to diligently buying from online duty-free shops.