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. If you accidentally swapped the labels for two orders, we can create shipping labels for the buyers to ship the books directly to each other.
Whatever the case, please reach out to our team with information about the issue and we will be happy to find a solution!