You set the postage - this was not a Simple Delivery label but a label you bought from Ebay.
It was up to you as the seller to check you were charging enough. Ebay's suggested postage prices are just suggestions and you are responsible for ensuring the items you sell are sent with sufficient postage.
It looks as if you paid for a medium parcel weighing up to 2kg and the item you sent weighed 4 times that amount. You should refund your buyer but you can't claim the money back from Ebay.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)