RE: Invoicing a line of a PackingSlip

You can try this:

salesFormLetter = salesFormLetter::construct(DocumentStatus::Invoice);

salesFormLetter.selectFromJournal(add Packing slip records here);

salesFormLetter.specQty(SalesUpdate::PackingSlip);

salesFormLetter.run();

Lines 2 and 3 are key to your requirement. Try in a Job first..