Feedback is voluntary and reflects the buyer's experience.
More important than feedback are the Detailed Seller Ratings the buyer left.
If any. Let's hope she was unaware of the importance to sellers of DSRs.
So far, your DSRs look okay.
You can add a Response to the neutral feedback referencing shipping time. Give the date of payment and of shipping. "Pd 12/15. Shipped 12/16. Recd 1/6 -10 working days" substituting the correct dates. Don't add Responses to most neutrals, it just draws attention to them. But that criticism needs to be pointed to as stupid.
The actual negative you got on shipping cost, I would suggest might have been avoided by using Calculated Shipping rather than Flat Rate. BDR. Buyers Don't Read. Calculated looks more official than Flat Rate.
If that buyer had complained about the cost after winning the bid, it is usually a good idea to offer to cancel the sale rather than pushing it. Better no sale than an angry buyer.