The checkbooks are being used to filter a recordset, then it is looping through the recordset to populate the cart contents. Once inside the recordset loop, it is not possible to match up the currant record with the correct quantity filed from the form collection.
The quantity being used is hard coded into tyhe add to cart behavior, it doesnt have to be 0, it can be what ever you select in the add to cart behavior for the default value of the quantity column.
Since the items are being added to the cart by looping through the recordset, not by looping through the form, it is impossible to link the currant record with the corresponding qty form element.