Hello RAH,

The macro is attached to the worksheet to the by its Worksheet_Change event procedure. Whenever a cell's value is changed, the macro runs. I have made a change to blank the quantity when the line is copied. The change is marked in blue.
Private Sub Worksheet_Change(ByVal Target As Range)

  Dim LastCol As Long
  Dim R As Long
  Dim vt As Long
  
    On Error Resume Next
      vt = Target.Validation.Type
      If Err.Number = 1004 Then
         Err.Clear
      End If
    On Error GoTo 0
    
    Application.EnableEvents = False
    
      R = Target.Row
      With ActiveSheet.UsedRange
        LastCol = .Columns.Count + .Column - 1
      End With
      
      If vt = xlValidateList And Cells(R, "A") = "Add a Line" Then
        Target.Offset(1, 0).EntireRow.Insert Shift:=xlDown
        Range(Cells(R, "B"), Cells(R + 1, LastCol)).FillDown
        Cells(R, "C").Value = ""
      End If
      
    Application.EnableEvents = True

End Sub
How to Save a Worksheet Event Macro
1. Copy the macro using CTRL+C keys.
2. Open your Workbook and Right Click on the Worksheet's Name Tab for the Worksheet the macro will run on.
3. Left Click on View Code in the pop up menu.
4. Paste the macro code using CTRL+V
5. Save the macro in your Workbook using CTRL+S

Sincerely,
Leith ross