Shortening Custom Document Property additions
Writing to a document property is a concern in its own right, and thus deserves its own scope. Write a procedure that's responsible only for this!

Public Sub IncrementHeavyDutyCount(ByVal increment As Long)
    With ThisWorkbook.CustomDocumentProperties.Item("numHD")
        .Value = .Value + increment
    End With
End Sub


And now you can increment your counter with a simple, self-explanatory procedure call:

If cell = "Heavy Duty" Then IncrementHeavyDutyCount 4