Similar set up in Belize. You ordered a litre of the local rum and six cokes for a fixed price (going back a bit so can't remember, think it was $3 for the 'One Barrel' Rum and $1 for the cokes) - If there was anything left in your bottle (You ordered one each) then the girls would get a felt tip pen, mark the level, you sign it and it gets put behind the bar.