Good catch about the class field being pulled from the invoice. However, the class field is not on the main line of the invoice, only on the lines. And on the credit memo it is in both spots. I'll check to make sure they know this is the case and if there is ever a time this field gets hidden from the form. then another field will have to be chosen.
I'll also ask if there is another field they'd rather it trigger from.
I'm trying to keep from scripting this as they specifically requested it be handled with a workflow...