They just tell you what the customer said.

you can:

Tell the customer that the item is no longer returnable, because the product was delivered more than 4 weeks ago and the return window closed and (I would only pick this option if this is true) the book is not resalable as a new edition may be released/ the text is time sensitive

Tell the customer that the item is no longer returnable, because the product was delivered more than 4 weeks ago and the return window has closed, but you are willing to accept a return for a full refund minus (shipping and fees or restocking fee)

Anything but accepting a free return with a full refund has the potential to get an A-Z opened that some rep will totally accept. You may then be found at fault, against their own policies. You will then need to proof your case, appeal, possibly appeal again and it may drag out a few weeks. Consider this when you reply to that customer. Now this doesn't happen as often as it appears, considering the million US sellers and seeing 1 or 2 of these a day, but you have to expect it.

Note, textbooks have extremely high "late returns" as it appears many use them for the full class and then open an A-Z when the seller wont take it back.....there are all sorts of issues selling text books on this platform and I believe some are even no longer allowed, not sure about that)

Selling Textbooks (policies)

When selling textbooks, it is important to understand the treatment of different editions, including previous editions, teacher editions, and international editions.

When listing your textbook on our site, the ISBN of your book must match exactly the ISBN on the book's Amazon.com detail page. International editions of textbooks must be listed against separate ASINs from the domestic product. (See Selling International Media Products.)

If the ISBN is not the same as the one on the detail page, you will be listing the wrong edition or cover-type of your book, even if the title and author matches. The ISBN, author, title, binding, publisher and date of publication must match the information on the product page or you may not list the textbook on that ISBN/ASIN.

Prohibited Textbook Items

Please take a moment to familiarize yourself with these descriptions of prohibited textbook items:

Edition Matched to the Incorrect ISBN: Listing inaccurate or incorrect book information or descriptions (including ISBNs) resulting in misrepresentation of the textbook. Examples include:

Wrong Edition: The book titles may match but not necessarily be the same edition. If the ISBN on the textbook does not match the ISBN on the detail page, you may not list the textbook against that detail page.

Incorrect Cover-Type: It is inappropriate to sell a softcover book against a hardcover textbook detail page or vice versa.

Copied/Unauthorized Material: You may not sell .pdf, scanned, photo-copied or otherwise unauthorized content as textbooks.

Promotional Instructor's Editions: Promotional copies of all media including books, music, and videos are prohibited. Typically promotional textbooks contain content that is identical to the Student Edition. The only difference will be markings or stamps displayed on the front or back cover, such as "Complimentary Copy," "Free Copy," "Instructor's Edition," or "Not for Sale." Do not list these items on Amazon.com.

Teacher's Editions and Annotated Instructor's Editions: Teacher's Editions are textbooks targeted to the teaching community. Annotated Instructor's Editions usually contain answers and/or instructor's notes added to the student edition of the book. In most cases, Teacher's Editions and Annotated Instructor's Editions are NOT intended for public sale and may not be listed for sale on Amazon.com. In a small number of cases, publishers intend certain titles to be available for public sale for self-teaching or homeschooling purposes. In general, if Amazon.com is selling a Teacher's Edition, it should be permissible for you to also sell that same Teacher's Edition. Otherwise, do not list these items on Amazon.com.

Solutions Manuals: Most Solutions Manuals are sold as supplements to a textbook and typically provide answers and descriptions to problems related to the textbook. Solutions Manuals that are intended for students may be sold on Amazon.com. Solutions Manuals that are NOT intended for students or for public sale may not be listed for sale on Amazon.com.