We are ordinary members here, not eBay staff.

Are you sure the invoice is real? Some invoices are scams/phishing emails where people are asked to click on links to go to sites that steal their details. 

Is this purchase in your purchase history too (does anybody else have access to your account - if not and the purchase is in your purchase history, your account has been hacked) or did you just get an email asking you to click on links? I hope you didn't click on any links if this is the case. If you did, change passwords and cancel credit cards (if you entered details).