If you buy from a business they should be able to 
provide you with a receipt stating how much the 
cel was sold for, and thus, that's what you will 
get back.

For transactions like those on eBay or between 
two people, I don't know what would be considered 
enough proof of value. Is a print out of the eBay 
page or personal emails sufficient? Surely the 
post office must have some standards regarding 
these types of insured packages where an official 
receipt cannot be produced. It may depend on how 
much was paid for insurance since you pay more in 
order to buy greater insurance.

---Trebuchet