As the seller, the first step is reaching out to the buyer and letting them know you’re aware of the mixup.

If you still have the correct book, we can issue a new shipping label for you to send it out and provide the buyer with a return label if necessary. In some cases if two orders were swapped, we can provide shipping labels to arrange a trade between the two parties. In most cases, however, it is much simpler and preferable to provide refunds to both buyers, which our support team can facilitate.
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Whatever the case, please reach out to our team with information about the issue including the affected orders and buyers, and we will be happy to find a solution!