If you have sent the buyer a message to ask if they would kindly change that feedback with a feedback revision you can send, nothing afterwards, to enforce them to do so.
To me it looks like they sent that to the wrong seller as they are complaining about paying £10 initially, and the item sold to them shows £5.99. Perhaps politely draw their attention to this.
If you get no joy from the buyer, contact Customer Services, draw their attention to tracking showing delivery and the obvious that this is sent to the wrong seller, as the costs of item received does not match the price of item sent.
However, I have just seen your follow up calling them a liar when I expect it was a genuine mistake sending to the wrong seller will have done you no favours for them changing that feedback now.
I do strongly recommend contacting CS very first thing on a weekday morning at 8 a.m.. when lines for ' live agents' open, and there's more chance of Dublin answering.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open for ' live agents' 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
As said, I recommend contacting CS first thing in the morning as there's more chance of Dublin answering, hope your follow up will not spoil this for you.
@xxshell86xx
