Does the price go down $5 for each additional item or does it just get a $5 discount on each bottle if there's more than one item?
If its a $5 discount on each bottle if there's more than one, you would apply the discount as a percentage and calculate how much $5 would be as a percentage. Then you would uncheck the require code checkbox. And put in the minimum purchase as higher than the single item price, but lower than the cost of 2 items. Then check the choose products checkbox.