As the seller, the first step is reaching out to the buyer and letting them know you’re aware of the mixup.
If only one buyer has been affected and 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.
Whatever the case, please reach out to our team via the support box or email at support@pangobooks.com with information about the issue including the affected orders and buyers, and we will be happy to find a solution!