If your book does not have an ISBN at all, then post it using the books without an ISBN feature.  Don't grab an ISBN for a book that looks similar and use that to post it.

Did you check the back of the title page?  Sometimes you'll find the ISBN listed there even if it's not on the outside of the book.

You should not have posted your first book under the Grade 3 ISBN, because your Grade 4 book is different.  You can post that one through the No ISBN feature.  As for your second book, since it has craft materials with it, it should not be posted.  It cannot be sent media mail with those.  You could probably offer it as a freebie though with an order (if you remember it cannot be sent media mail).

Help Center sez:

• When do you need to Post a book using the Post Books Without an ISBN feature?
• When your book has no ISBN
• When your book's ISBN is unknown to the PBS database (has not yet been added by regular updates)
• When your book's ISBN "brings up" another booktype in the database
  • Such as the paperback if you have the hardcover, and vice versa
  • Such as Large Print when you have the regular-font version
  • This happens when the publisher assigns the same ISBN to multiple booktypes).

If your book does not match the listing information (title, author, booktype) in the database for the ISBN on the book, it must be Posted without an ISBN

Does the craft book have craft supplies or does it just provide the projects?  If it's just the projects I believe it can be posted. I posted a kids craft book that was WL. But it was just the directions for the crafts.  No cutouts or supplies came w/the book.  Just the ideas and directions. It has an ISBN#.

I think the craft book would be fine, but if you're really concerned about it, you could always send it first class.  That would rule out any question of whether it's acceptable as media mail or not.

If  any book has no ISBN or the ISBN is not matching the PBS system, do not bother to search for it on Amazon (or any other site) for the ISBN, you CANNOT post it by that number here. You must post it as a no ISBN number on PBS.

I'll second what Melanie & some others have said here.  If a book doesn't have an ISBN, you don't try to look it up.  There are often many editions of a book, each with a separate ISBN.  They are not interchangeable.  If a book doesn't have an ISBN, the ONLY way you can post it here is to "post without an ISBN" at the bottom of the "Post Books" page.  (The Post Books page also has suggestions as to where to look for an ISBN on the book itself.)

If, say, it's a wishlisted book, and you are concerned that people who want it won't know about it, you can always go to the Discussions Forum (under "community") and mention it in the Book Bazaar.