Hi Mark,
This is just a quick guess, but usually when a field has predetermined values (that you can add to or not), with the DI API you can only assign an existing value, and when you assign it, B1 can be very uptight about how you write it. In other words, if the existing value is "Carton" (capital C), then it may not work if you try to assign "carton" (lower case c).
With quantity type fields, B1 can be similarly uptight about the format of the numeric value you try to assign. For example PurchaseQtyPerPackUnit has the format numeric(19,6), which is not an integer. You could test assigning specifically a value with a decimal. So instead of 1 try 1.0
Regards,
Johan