Why not use Attributes and let the customer type in how many they want.

Then they could still use the quantity to determine how many containers they wanted ex: 2X5L or 7x3L.

Otherwise, have various Litre points as Att. Options .. 1L, 2L, 2.5L, 3L 4.25L
You CAN allow a custom customer supplied entry here (empty text field)

So instead of changing the qauntity setup, try to use attributes, you'll find that the built in price adjustment and features work well.

Then if you still don't like the quantity field being there - you could hide it with css, or making a custom node-product.tpl.php file.