The fee IS by unit, not volume ....

Tell me if you think this is swingable:

You can bring in your own wine to Venue X for a $Y per bottle corkage fee.

Boxed wines (now in non-horrible varieties!) come in five liter units. That's equivalent to 6.67 bottles.

Could you successfully argue that the corkage fee for a box should be $Y, not $6Y?