I am going to send a diamond (valued $7500) to my sister who is living in WA (I am in PA). I went to FedEx and the guy said the maximum FedEx express covered is $1000. So how to send the stone with fully insured?
A claim with the USPS requries proof it is worth what you insured it for.
They don't take your word for it.
A reciept or appraisal will work.
Don't ship it if you don't have this proof.
Also prices have gone up a lot in recent years, so if you bought it 5 years ago for $7K it may be worth twice that now.
Expect it will take 15 days regardless of what the USPS clerk tells you.
Also don't pay extra for faster deliver regardless of what they call it, priority, express, next day, 2-day whatever.
Offering those fast services is a scam if it is Registered since ALL registered mail travels the slowest way and will take a long time, up to 15 days.
If it gets there any faster consider that a nice surprise.