Packaging is dimes a unit for liquid that is pennies a unit to manufacture. Bulk sales normally are discounted because more unit sales is pretty much always better than more money per unit. This is trivial to prove and us true as long as Pnew-Pold > Pold, literally as long as you aren't doubling the price, one more unit sale is always better than having increased the price on one unit.

Also bulk packaging is literally cheaper per unit. Companies which do the opposite, charge more for bulk orders, generally are doing so out of the expectation that customers expect the bulk option to be cheaper and are thus tricked into being "double dipped", they have both spent more per unit and bought more units.

Why does the party cost money? I'm assuming you mean extra money, but I'm guessing after parsing that.