The brand may just be using the book as backwards advertising. Bundling the products and saying that Ayu used them to sell units.
They may have paid, but they could also just be cashing in on them being featured.
The brand itself looks really nice and natural, I'll be trying them out once I've used up the stuff I have.
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"Remember, don't let others dictate your music taste. If you like whatever you're listening to, keep listening to it."