Do you still have the orginal pink slip? If so it has the tracking number on the package/letter. You could use that to contact Disney and possibly have the package/letter re-sent.
Here's the procedure for delivery of a certified letter/package.
Just prior to leaving the office for delivery, carriers are brought their "accountables" which they sign for. These include registered, certified and insured items.
The carrier then fills out a pinkish/peach slip known as a 3840. It has your name, address, the tracking number, what it is (letter,parcel) and what type of accountable it is,date and carrier intials.
When the carrier gets to the house he/she has to scan the bar code on the letter/package. They attempt to deliver it ( you have to be there ) if you aren't home, the 3840 is scanned as "attempted" and then placed in your mailbox.
Upon return to the post office, the scanner is put in it's charger and the information that was scanned into it, downloaded.
The letter is then handed back to the accounting clerk who is then suppossed to lock it up until:
A-you come sign for it
B-2 3840's have been delivered to your home and at least a week has passed.
If you don't come the first time, 3-4 days later a 2nd 3840 is given to the carrier to deliver. The accounting clerk writes these up.
How they could lose the letter/package is a shocker to me. The only time it leaves the safe is when it is picked up or sent back.