Wrong Sized Bed Sheet

I am so mad right now! Couple weeks before I bought a new fitted bed sheet for my mattress. My mattress is 140cmx200cm. I asked the sales assistant which size bed sheet I need to buy to fit to my mattress. Se told that I should buy 140x200 as there is 25cm excess width of fabric to each side.
Yeah, right! I opened it today and it even does not cover my mattress, not talking about additional 25cm. Where the hex is the 25 centimeters if I even can't cover the mattress top?
I can't return the sheet to the shop for two reasons. First, I do not have the receipt any more. Second, it is past the time, I could return the item. It is more then 14 days already.
That is the reason I like shopping in internet. To return an item to internet-shop, I just need to go to local post office. In my case I have to drive to another town to return the sheet to the merchant.
P.S. The sheet wasn't the cheap one!