The first thing I would be doing in this situation is contacting the buyer to try and resolve the issues they have.

It would have been good for the buyer to have contacted you with their concerns but often buyers don't take the time to do that.

If the postage cost was $1.80 plus the cost of a bag and maybe a couple of dollars for handling then you could offer a partial refund.

I would also clarify the problem with the address as they are claiming the parcel was sent to the wrong address.
You need to find out if that was your error or the buyers error.

If you are able to sort the issues out with the buyer then you could ask them if they would be willing to revise the feedback they left.

At this stage, from the buyer's perspective, it has not been a positive experience. It is now up to the seller to see if they can turn it around into a more positive experience for the buyer.

Many buyers don't take the time to contact sellers first as they think they won't do anything about it. I would be taking this opportunity to let the buyer know that when there are issues more often than not the good sellers are more than willing to try and resolve an issue.