We've always allowed Global students to return books and physical items by mail by sending a postage-paid mailing label along with their items and have extended this service to all WSU employees and students.
If you return library material in a box, pack the box tightly so the material isn’t damaged and doesn’t break out of the box during shipping. You can also use a padded envelope for smaller items. If you received the item via mail from the WSU Libraries, keep the package that it came in and use that to send it back.