“if you want to be מזכה people donate it to a beis Medresh”
maybe he is worried that if he donates it without charging it will show he isnt machshiv torah, which apparently is a view some have.
So to cover all bases he is charging something to show that it is chasuvh but not too much so its still affordable.
Anyway, on to Tachlis, I love buying seforim at a cheap price (probably because I too am not machsiv Torah I guess ) where can we get them?