You are not responsible for where a postie / courier leaves an item, by bins, seems to be a rgular spot.
Do you have tracking to show delivery? That is all you need to enter, if the buyer opens a case for item not received, the case will then close in your favour.
If you do not have that tracking, you will have to fully refund your buyer, do so only through a case the buyer needs to open.
Either way, they're a troublesome buyer, so block them from returning to your sales here:
https://www.ebay.co.uk/bmgt/buyerrequirements
@johnlydon77