I sold an item late on Wednesday evening (1st).  The item was on 2nd class postage with a 3 day handling.

The buyer opened a case for non-receipt on Sunday (5th)

Could you please explain how this is possible?

Can you offer advice as to what settings I should use to avoid this happening again as we are often unable to post for a few days - hence the 3 day dispatch time?

As the buyers feedback was less than inspiring, we had in fact posted the item tracked on Thursday, and the item had been delivered on Saturday , tracking was uploaded to ebay and showed delivered.   

Can you explain how a buyer is able to open a non-receipt case when the item has been delivered?  The buyer may have issues with the item and maybe want to open a case for other reasons, but it should be obvious that they should not be able to open a non-receipt case.