Hello iftikhar,
<<cell phone magnet holder . . . I did not receive the item in prescribed delivery window. >>
That sounds like a cheap little item bought from China. Even when they are mailed immediately they often take 6 weeks or more to arrive, as they usually come by boat. But this, -
<<ebay opened my case and seller sent info to me that they will send the item.>>
What do you mean ebay opened your case? Did you yourself open a claim for Item Not Received? That is what you want to do when an item does not show up.
If, after a claim has been opened, the seller says he will send the item, - that is too late. It means he did not send the item after you paid, and that means it is time to get your money back.
When a seller does not send an item, you want a refund. You do not want a promised replacement item because those are never mailed either. Sellers make this promise to get you to close your claim.
A closed claim cannot be reopened and you will lose your money and get no item, -- so never close a claim until you get your money back.
If you try to plead with the seller that you still want the item, he will tell you what you want to hear, get you to close the claim, and again you will get nothing.
<<...need acknowledgment from ebay that my case has been prcessed properly and my case is not closed without solving.>>
When you open a dispute the seller has 3 days to respond. Right now you have nothing, so if you have not already done so, escalate to a claim for a refund. If the seller refuses to give you back your money, ask ebay to step in.
Get your money back and next time find a more honest seller. Do not buy from this one again.
Best of luck 
http://pages.ebay.ca/help/buy/item-not-received.html#notreceived
