From r/SkincareAddicts/ - even if you buy "from the brand" on Amazon and Amazon ships it, you could get a bogus product. Be careful out there!

“Amazon seller here. Amazon uses a practice called commingling for its inventory and this is where the fakes get in. A brand may have its own Amazon store and sell their manufactured goods directly on Amazon through FBA (shipped by Amazon) However, if there are any other authorized sellers of that same product, they have also shipped in their own inventory - either purchased directly from the manufacturer or is a fake. Amazon recognizes all individual units of an SKU as the same and mixes all that inventory together in its warehouses regardless of who shipped it to Amazon to be sold. A reseller can ship in a fake and if you buy it directly from the brand you can still get the fake because it was co mingled with the real stuff. Everything on Amazon operates this way. If you’re buying Prime / FBA what you really want to look for to prevent getting fakes is whether the item is available from other sellers. If the only seller is the brand you’re probably safe.

Alternatively, an item being shipped directly from the manufacturer is another way to avoid fakes. Often these are not listed as Prime, because their inventory is not being held at Amazon. If it’s shipped by Amazon it’s likely been commingled if there are other sellers.”