Do the sort in steps:
Sheets("Shipping").Range("ShippingDetailedItems").Sort Key1:=Range("D8"), Order1:=xlAscending
Sheets("Shipping").Range("ShippingDetailedItems").Sort Key1:=Range("A8"), Order1:=xlAscending, Key2:=Range("C8"), Order2:=xlAscending, Key3:=Range("D8"), Order3:=xlAscending
Kevin