Add /TAX item to Invoice

Hello, I have a customer who wants to have the Tax appear as the line item /TAX on an invoice. 

Here is what I'm inserting after the items get added:

retVal = oSOInvoice.oLines.nAddLine()
retVal = oSOInvoice.oLines.nSetValue("ItemCode$", "/TAX")

retVal = oSOInvoice.oLines.nSetValue("ItemType$", "5")
retVal = oSOInvoice.oLines.nSetValue("ItemCodeDesc$", "")
retVal = oSOInvoice.oLines.nSetValue("SalesAcctKey$", "")
retVal = oSOInvoice.oLines.nSetValue("Quantity", 1)
retVal = oSOInvoice.oLines.nSetValue("Price", 1.65)
retVal = oSOInvoice.oLines.nSetValue("ExtensionAmt$", 1.65)

retVal = oSOInvoice.oLines.nSetValue("Cost", 0)

retVal = oSOInvoice.oLines.nSetValue("WarehouseCode$", "")
retVal = oSOInvoice.oLines.nSetValue("UnitOfMeasure$", "EACH")

retVal = oSOInvoice.oLines.nSetValue("TaxClass$", "NT")


retVal = oSOInvoice.oLines.nSetValue("DropShip$", "N")

retVal = oSOInvoice.oLines.nSetValue("QuantityOrdered", 1)
retVal = oSOInvoice.oLines.nSetValue("QuantityShipped", 1)
retVal = oSOInvoice.oLines.nSetValue("UnitPrice", 1.65)

retVal = oSOInvoice.oLines.nSetValue("Discount$", "N")

retVal = oSOInvoice.oLines.nWrite()

Some the stuff there is a swing in the dark, so it could be wildly inaccurate. 

Any ideas as to why this line item won't show up on the invoice?