Since you're starting by creating oLines, your script trigger must be in the header already, which doesn't have oHeaderObj... use oBusObj for your SetValue lines.

Thank you and that worked really well for the Sales Order Header, but I tried the same logic for the Invoice Header and the Weight calculates properly, but does not update in the Invoice Header. I have updated the script again to check that the field name is correct and I removed the default from the Sales Order Header, but I cannot get it to work.

TotUnits=0
Totwgt=0
itemtype= ""
Set oLines = oBusObj.AsObject(oBusObj.Lines)
retVal = oLines.MoveFirst()
Do until oLines.EOF
retval = oLines.GetValue("ItemType$", itemtype)
if itemtype = "1" then
retval = oLines.GetValue("UDF_PALLET_COUNT_UNITS",punits)
retval = oLines.GetValue("UDF_PALLET_WEIGHT",pwgt)
TotUnits = TotUnits + punits
if pwgt = 0 then

Totwgt = Totwgt + (punits * 75)
else

Totwgt = Totwgt + (punits * pwgt)
End if
End if
retval = oLines.MoveNEXT()
Loop
msgbox(Totwgt)
retval=oBusObj.SetValue("UDF_PALLET_WEIGHT LBS",Totwgt)
retval=oBusObj.SetValue("UDF_TOTAL_PALLETS_UNITS",TotUnits)

I think I am just looking at too many numbers. I think it is working. Thank you.