Yep, I have also received the refunds on missed Express Mail delivery guarantees. In fact, they are so bad at getting things delivered on time, they MUST be loosing money on the service... if only more customers cared to settle up with them for their failings! Their inconsistency, just isn't worth my time to chance showing up with a box that might be rejected for Registered service, despite the customer being able to quote and instruct the postal staff on their own policy.
They did get really bad at that last year, I suspect that's why they raised the insurance rate to the same as priority when it used to be cheaper.
But all of those are still just "Registered" mail and all of them cost the same and are shipped the same way and at the same speed. It isn't a secondary service. Once you say Registered it is just Registered mail. So insist they prove it, without unwrapping it. The box sizes are not unique to the postal service.
Easier said than done. Once they refuse to do their job that's the end of it. No service for you! Once they refuse to serve you on unfounded claims they won't care what justification you have and soon customers waiting in line behind you will start complaining that you're holding up the line. Almost all New York branches have thick glass between the clerks and customers which I think emboldens their behavior and thus requires them to hide behind the barrier for fear of what some people might do for screwing with customers in such petty ways.
Is this a dealer a friend or someone you trust? If so how about just overnight it signature conformation (no ins needed) OR If this isn't a time sensitive transaction trip adviser says it's 208 miles and a 3 1/2-4 hour trip pick a day/time, meet in the middle, bring a friend, meet in a safe location transaction complete (no ins needed).