"Add to inventory?" prompt if item not found

If you want the message box from post 3:

Private Sub CommandButton1_Click()

Dim AWF As WorksheetFunction:   Set AWF = WorksheetFunction
Dim MatchRow As Long:           MatchRow = 0

On Error Resume Next
MatchRow = AWF.Match(Range("A6"), Range("A12:A1500"), 0)
On Error GoTo 0

If MatchRow <> 0 Then
    Range("C" & MatchRow + 11) = Range("C" & MatchRow + 11) + Range("B6")
Else
    Dim Msg
    Msg = MsgBox("This part number is a new part number. Do you want to add it to the parts list?", vbYesNo, "Confirm")
    If Msg = vbYes Then
        Range("C" & Range("A" & Rows.Count).End(xlUp).Offset(1, 0).Row) = Range("B6")
        Range("A" & Range("A" & Rows.Count).End(xlUp).Offset(1, 0).Row) = Range("A6")
    End If
End If

End Sub

Regards