You would get a more definitive answer if you go on to QandA on Ebay
In my opinion the previous 2 answers are wrong as a number of factors have not been mentioned.
I can assure you that you are correct that it is the sender's responsibility.
If you paid by paypal then start an Item Not AS DESCRIBE dispute...via paypal...you will win.
If you paid by Credit Card for over �100 you can do a chargeback
If you paid by po or cheque start the same dispute via ebay.
Insurance is for the sellers benefit, not yours.
They have the contract with the post office, not you.
Good luck