I would suggest not to refund your original ship costs

When you say your selling a first edition book make sure you know how to confirm a first edition or not, and also understand what First edition really means. This makes all the difference to a book collector the phrase 'First Edition'. It refers to a book that was released during the first printing of the 'First Edition', commonly referred to as a first/first.  A first/first is what collectors are seeking.
Here is how to know
At one time, books simply stated 'First Edition' on their Copyright Page.  On older books printed before the advent of ISBN's (about 1968), check whether the book was printed the same year that the book was first published. Newer books printed after 1968 may also have a number sequence printed used until the 1980's.on the Copyright Page that determines its printing history.
A number sequence that has a "1" at the end such as "10 9 8 7 6 5 4 3 2 1" indicates a first printing.  This is typically referred to as, a full number line.  But if the lowest number showing in this number line is a 2, then the book is a second printing, and not as valuable to a collector. Keep in mind that the number line method was not widely used. It is also know as, "The printer’s key", or also known as the "number line", is a line of text printed on the copyright page of books, used to indicate the print run. Publishers started to use this convention around the middle of the 20th century.
An example follows:

1 2 3 4 5 6 7 8 9 10
This is how the printer's key will appear in the first print run of a book. Numbers are removed with subsequent printings, so if "1" is seen then the book is the first printing of that edition. If it is the second printing then the "1" is removed, meaning that the lowest number seen will be "2".


Usually it is a series of numbers or letters as in the following examples:

10 9 8 7 6 5 4 3 2 1
1 2 3 4 5 6 7 8 9 10
a b c d e f g h i j k
Sometimes rather than follow in series the numbers alternate from left to right for example:

2 4 6 8 10 9 7 5 3 1
The purpose of this arrangement is to keep the line of numbers roughly centered even as the numbers are removed with subsequent printings. This format is seen in the copyright page image on this page.

Sometimes number lines will also include a date line for example:

2 3 4 5 6    73 72 71 70
This indicates a second printing (or second impression) that took place in 1970. More specifically, it is this particular imprint's second impression of the edition.

Sometimes, when the publisher outsources the printing to a contractor, there will also be a code for which printing company was contracted in this case:

3 4 5 6 7 8 9 10   APC   00 99 98 97 96
The hypothetical printer’s key above decodes as third print run, printed in 1996, contracted to Acme Printing Corp.