I think Freight are additional expense in header. You may try the code below:
Dim oInvoice As SAPbobsCOM.Documents
oInvoice = oCompany.GetBusinessObject(SAPbobsCOM.BoObjectTypes.oInvoices)
With oInvoice
.CardCode = "Customer Code"
.DocDate = Date.Today
oInvoice.Lines.ItemCode = "ItemCode"
oInvoice.Lines.Price = 100.0
oInvoice.Expenses.ExpenseCode = 0
oInvoice.Expenses.LineTotal = 100.0
oInvoice.Expenses.Add()
End With
You can get the expense code here: OEXD