Due to my location I had a set shipping amnt for items.. example 10$. In my listing I stated that any portion of shipping that was not used would be refunded. Never had an issue and its simple to do!
Lastly the only way I would leave a negative FB is if he would not refund you.
If they are willing to refund and make you whole then I see no reason for a neg..
The righteous pay a sacrifice to get what they deserve.