Which negative are you querying?
The one asking for combined postage? It's not too late to send the buyer a polite message to partially refund that difference, however, they are also claiming the items arrived damaged, so inform them to open a case for item not as described, and you can fully refund them.
Keeping them sweet, keeping the messages polite, if the matter is resolved with the refund, you can then ask the buyer if they would kindly complete a feedback revision you can complete.
If it's the other feedback, unfortunately if this damage was only shown in photos, this is not sufficient as this can easily be missed especially viewing on a phone. Any damage should be clearly described in the description box.
Again here, you may redeem the situation by apologising ( I agree this sometimes needs to be done through gritted teeth) perhaps offering a partial refund then you can ask if they would complete a feedback revision too.
Neither feedback breaks feedback rules so eBay will not remove them.
@cat0112
