I was able to retrieve the customer packing slip journal record and pass it to the SalesFormLetter_PackingSlipCancel action menu item.
CustPackingSlipJour custPackingSlipJour;
boolean isCancelEnabled, isCorrectionEnabled;
Args args;
//find latest packing slip
select * from custPackingSlipJour
order by PackingSlipId desc
where custPackingSlipJour.SalesId == salesTable.SalesId;
[isCancelEnabled, isCorrectionEnabled] = custPackingSlipJour.canPackingSlipBeCanceledOrCorrected();
if(isCancelEnabled)
{
//"cancel" latest packing slip
args = new Args();
args.record(custPackingSlipJour);
new MenuFunction(menuitemActionStr(SalesFormLetter_PackingSlipCancel), MenuItemType::Action).run(args);
}
super();