Use a product option.
Create a new column for CaseSize. In the add to cart behavior, set the CaseSize column to be updatable as a select list. Populate the select list with the options:
then create a new calculation in the eCart object calculations tab:
(abs([CaseSize] == '6 Bottles')?[Price] * 6:0) + (abs([CaseSize] == '12 Bottles')?[Price] * 12:0)
then edit the TotalPrice calculation to:
((abs([CaseSize] == '6 Bottles')?[Price] * 6:0) + (abs([CaseSize] == '12 Bottles')?[Price] * 12:0)) * [Quantity]
NOTE: my calculation is based on the name of the column being CaseSize and the select list values being 6 Bottles and 12 Bottles, if you use different names / values, you will need to modify the calculation accordingly.
in the cart display, change out the Price binding for the TruePrice binding, and in the code that passes the transaction to the gateway.