I am new and need help.  I sent out two books on the same day and mixed up the labels, so the books went to the wrong people.  What can I do to make sure the books get where they are suppose to be with no cost to the receivers of the wrong books?  Thanks.

You need to PM these people and let them know what happened. The easiest thing to do, if possible is to give them each their credit back.  Then let them know who their book went to and have them order the right books off eachothers shelf.  Or you can give them each 2 credits and they can send the books back to you.  I had this happen to me recenlty.  The sender gave us each a credit, I ordered my book for a 2nd time from the person she sent it to and that person ordered their book from me.

I think this has happened to a lot of us. I know it has to me. If you warn the receivers that it is the wrong book they can refuse it and it will come back to you, then you could mail it again. Other alternatives are as Mary said. But usually if you offer a credit for them to post the book forward that will work. Best scenario is for them to refuse it.

Note that if you sent it media mail, you should be (but might not be) charged postage for the return.  If you sent first class, then return postage on refused items is included.