A saw another ‘Bad Travel Hack‘ this week. . . You can take a bigger bottle of shampoo in your carry on . . . if you buy it in the terminal, after security!
Have you seen the prices!
It is not just the pharmacies, pretty much all the shops at the airport are much more expensive.
I have seen products in the airport shops 300% or more above the price in our local supermarket.
Another problem with this suggestion is you might have problems if its not a direct flight.
Travellers Hack
Try buying the shampoo at your destination.
You may be surprised at the variety in supermarkets at most travel destinations and its likely to be cheaper than you could buy at home.
