1st bad feedback and not my fault! how+why is this possible

[owenuk2013]

Hi,I recently posted a item to a customer Royal mail 2nd class,customer said they didn't receive the item an opened a case with ebay which then they was refunded the full amount.
Then the customer leaves me bad feedback for not receiving the item.
1 How is that my fault,I posted the item an have proof of postage.now I have to claim my money back from royal mail an not only that I was left bad feedback because the customer didn't receive the item.do people not realise that after I post an item it is out of my control if they receive it or not an how an why dies ebay let them leave negative feedback when its clearly not my fault,I put I will post within 2working days an always post after 1 as well.this is so annoying and upsetting that ebay allow the customer to do this.