Combine specific line items on invoice

I'm not sure if I understood correctly but try this macro on a copy of your workbook.

Sub SumItems()
    Application.ScreenUpdating = False
    Dim LastRow As Long, InNum As Range, Count83 As Range, Count75 As Range
    LastRow = Cells.Find("*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
    Sheets("Sheet1").Range("A1:A" & LastRow).AdvancedFilter Action:=xlFilterInPlace, CriteriaRange:=Range("A1:A" & LastRow), Unique:=True
    Set rnguniques = Sheets("Sheet1").Range("A2:A" & LastRow).SpecialCells(xlCellTypeVisible)
    For Each InNum In rnguniques
        Range("A1:R" & LastRow).AutoFilter Field:=1, Criteria1:=InNum
        Set Count75 = Range("F2:F" & LastRow).SpecialCells(xlCellTypeVisible).Find("UN0000075")
        Set Count83 = Range("F2:F" & LastRow).SpecialCells(xlCellTypeVisible).Find("UN0000083")
        If Not Count75 Is Nothing And Not Count83 Is Nothing Then
            Range("L" & Count75.Row) = WorksheetFunction.Sum(Range("L" & Count75.Row) + Range("L" & Count83.Row))
            Rows(Count83.Row).EntireRow.Delete
        End If
    Next InNum
    Range("A1").AutoFilter
    Application.ScreenUpdating = True
End Sub

Could you please attach another copy of your file highlighting the data/rows that were missed with a color so that they are easy to see?

Try:

Sub SumItems()
    Application.ScreenUpdating = False
    Dim LastRow As Long, InNum As Range, Count83 As Range, Count75 As Range, rng As Range
    LastRow = Cells.Find("*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
    Dim RngList As Object, Key As Variant
    Set RngList = CreateObject("Scripting.Dictionary")
    For Each InNum In Sheets("Sheet1").Range("A2:A" & LastRow)
        If Not RngList.Exists(InNum.Value) Then
            RngList.Add InNum.Value, Nothing
        End If
    Next InNum
    For Each Key In RngList.keys
        Range("A1:R" & LastRow).AutoFilter Field:=1, Criteria1:=Key
        Set Count75 = Range("F2:F" & LastRow).SpecialCells(xlCellTypeVisible).Find("UN0000075")
        Set Count83 = Range("F2:F" & LastRow).SpecialCells(xlCellTypeVisible).Find("UN0000083")
        If Not Count75 Is Nothing And Not Count83 Is Nothing Then
            Range("L" & Count75.Row) = WorksheetFunction.Sum(Range("L" & Count75.Row) + Range("L" & Count83.Row))
            Rows(Count83.Row).EntireRow.Delete
        End If
        Range("A1").AutoFilter
    Next Key
    Application.ScreenUpdating = True
End Sub

You are very welcome and thank you for the rep.