Getting negative feedback is not a nice feeling.
It is possible to get negative and neutral feedbacks revised.
It does involve saying sorry even when you are not at fault. This is an art. The first negative is the worst.
My advice is to do nothing on the day they leave the feedback. A good nights sleep does us all good. They were likely to be in a bad mood when they left the feedback so a night or even two nights will help.
I would email buyer as a first step. Emails are an easy format to control what you want to say and is easier to stay calm. Telephone as a last resort. Explain what has happened. Tell them what you could have done and what you are going to do in the future to make your business better. Resolve any issues where possible.
Explain how this negative or neutral feedback affects you and your business and ask if you would be prepared to revise the feedback that they left.
You then issue a feedback revision request and pray. You can find this form in the feedback forum area and clicking the link on the right hand side.
More often than not the buyer will revise the feedback as you have shown a good, helpful attitude and they are feeling better!