How do i resolve this international "where's my item?" case on Ebay?

i started selling to the uk and started running into some problems. at the beginning of February an item was purchased and shipped to lithuania via u.s. postal service. i have the declaration receipt to prove the item was sent and to the proper address. i am aware it takes 15-20day sometimes a good month especially going across europe. i explained this to them and they opened a case anyway. i have a few days to resolve the problem and all i can say is calm down and wait. From what i was told the customs receipt is enough sellers protection but i don't even know who i would summit it to....a little advise please! Just a little in over my head on this one....How do i resolve this international "where's my item?" case on Ebay?
This is why I do not ship outside of the US or Canada. If you got paid through Paypal, Paypal requires not just proof that the item was sent, but proof that the item was signed for (or delivery was attempted) at the receiving end. Shipping to anywhere in eastern Europe is risky for the seller. Not only are postal services unreliable, but some buyers from there are notorious liars. Your only hope is if the item was insured for full value.