How do you cancel a transaction?

A buyer purchased something from me and wants it shipped. I went to the post office, they scanned the package, but then said the weight on the label isn’t enough. How do I cancel the transaction or edit it to change the weight of the item? The status says “shipped”, even though it hasn’t yet.
New Member
Well u got to let the other person know he can cancel the payment since u don't want to cover the cost to ship the item since he didn't know that you didn't add the correct shipping rate