, I have the postal reciept with date, time etc,

You will need more than that, at the very least you will need to have either the buyer's postcode on the PO receipt and a tracking number to show that the item hss been delivered.  A receipt without a postcode just shows you have posted something, it could be a pressie to your aunty Marge for her birthday for all ebay would know.

If you do not have tht level of proof you would have been better off just refunding the buyer straight away, you may then have avoided the neg yoou will almost certainly get.