Sales Invoice from CustPackingSlipJour form

Hi, I’d like to add the functionality for users to create/post an invoice from the CustPackingSlipJournal form for that selected packing slip. I’d like to use the SalesFormLetter_Invoice menu item so they get the SalesEditLines form just like they do when they post from the sales order form. Any help is appreciated!

Why cannot you use the existing functionality of posting an invoice by selecting the packing slips (click select packing slips on invoice posting form)?