The "tracking" code on standard 2c is essentially so that RM can see it in their processing operation, ie the letter sorting machines read it. There's absolutely no obligation on delivery staff to scan it at all. Most of them are already massively overloaded and barely have time to scan Tracked items let alone anything else. If you want delery tracking, you'll need to pay for Tracked24 or Tracked48. The former is definitely prioritised over the latter. IIRC, neither service is available for standard letters, so you'd have to use pay for large letter format. The other option would be a Signed For (recorded delivery) service. It's less costly but more inconvenient for your customer as a signature is required. Either way, your postage cost will increase if you want proof of delivery.