@Doomer: Reminder:
1. She's from a poor family.
2. She doesn't care about profits, it's Benno who's been pounding it into her to care (Myne just want books)
3. None of her family members ever ran a business, her dad is a soldier and her mom is a worker not a store owner.
So with all these in mind, do tell, outside of some ass-pull development, how would a single family with no connection to the rich, or even registration with the merchant guild, run a business to sell shampoo? (Heck, Myne and Lutz get their registrations done basically thanks to the hairpin and Benno using it as bait)
The shampoo he'll be making will be of much better quality/varieties, with materials that Myne's family will likely never get access to normally.
It's not like Myne will be making any shampoo for his store, he's only buying the rights, the actual making/hiring workers/workshops etc will also be out of Benno's pocket.
And again, he's paying for basically everything the two spent for at least 6 months, and the magic contract (again, this thing alone is EXPENSIVE, not only is the material to make it very rare, just having access to it is in a way a show of how well your store is doing)
All of that and he doesn't keep the right to paper and for each paper sold Myne get cut of it as royalty, so it's not like Benno keep all the money here.
Also, all these are just to RESEARCH making paper, not mass producing them.
He's BUYING all the papers the two make until their baptism, after which they can officially start mass producing it for real (as in, making new set of tools for adult size, hiring more people, securing a workshop location, etc, all of which, again, will be on Benno's)
Seriously, taking a step back to look at it, he's investing in a project that has no guarantee to success on the words of a kid from the poor district and with them keeping the rights.
This mean that if at the end of this Myne and Lutz decide to sell through another store, Benno's investments would go to benefit someone else, since because of the magic contract there's absolutely no way for him to get papers without Myne/Lutz's consent.