I sell unit or a case of 50 unit, How can I control the Stock by Attribute?

in my store, a typical item is available in unit or a case of 50, some are available in unit and a case of 20,30...etc.

My problem is, I add the Package attribute with 1 or 50, however, how can zen-cart take care of my stock after buying?

thank

(either seperate stock for attribute is good, or a mixed stock is good, even a stock that only count the case that left is accepatble.)

In the attributes you can have attribute values and call these case50, you could have case 20, case30 etc all under the attribute name "Case"

Keep the unit as one and define the case qty in the factor & offset for these.
When you get this correct and pricing correctly it will also adjust the inventory correctly