I'm trying to post the Mass Market Paperback of Mary Balogh's The Arrangement.  The ISBN on the book leads to the details of the hardcover book.  I looked both versions up on Amazon and the hardcover has a totally different ISBN than the one on my PB.  How do I go about getting this changed?  I don't want to post the book and have someone be unhappy with it because it is a different binding.

My book's ISBN is 978034553587  (details from Amazon)

• Mass Market Paperback: 400 pages
• Publisher: Dell (August 27, 2013)
• Language: English
• ISBN-10: 0345535871
• ISBN-13: 978-0345535870
• Product Dimensions: 6.9 x 4.3 x 1.1 inches

• Series: Thorndike Press Large Print Core Series
• Hardcover: 511 pages
• Publisher: Thorndike Press; Lrg edition (January 29, 2014)
• Language: English
• ISBN-10: 1410456072
• ISBN-13: 978-1410456076
• Product Dimensions: 8.6 x 5.6 x 1.2 inches

this one: 978-0345535870 brings up a hardcover in the PBS system.  Since there are 4 copies of this book posted into the system PBS is not likely to change it from hardcover to mmp.  They rarely change that kind of info on a book and they'll only do it you can prove the book never existed in that format and no one has ever traded it or posted here. But 4 people have it posted under the hardcover listing. (you can tell that by putting it on your reminder list). I suppose the 4 people with it posted could just not have paid attention to the listing. You could try submitting a data edit and ask the team to look into by sighting all the places that show it as a paperback. It's possible that they put out a book club edition that shared the mmp ISBN. I've had book club editions with that issue before but it was the opposite where I had the hardcover and the ISBN brought up the mmp. 

Thanks....when I received the book I didn't even realize the ISBNs were different and had messaged the person who sent it to me about being more careful when they post since I had the hc on my wishlist, but now I'm thinking that they didn't double check what the binding type said when they entered it and I'm guessing that all 4 copies in the system are really the same as my mmp and are just listed under this hc entry.

Taryn, you can't really assume that, unless you've done a lot of research and haven't found anything else with that ISBN. 
As long as there are copies posted to that listing, it is very unlikely to get changed.  You can always post yours without an ISBN,
since you can't post yours with that ISBN as long as the listing remains as it is.  Per the Help Docs: 

If your book does NOT match the information in the Book Listing Preview:

• If it is a minor difference, such as a misspelled title or the wrong author first name, you can post the book and submit corrections afterward using the Edit Book Data link.
• If the booktype is different (if your book is a paperback, and the Preview shows hardcover, for example):
  • You will need to post the book without an ISBN
• If the book title says "(Large Print)" and your book is regular-size font:
  • You will need to post the book without an ISBN

FYI: the book was originally supposed to be published in hardcover, but was publication was postponed and they reused the ISBN for the paperback.

I knew about the change in formatting, so when I got my copy, I was happy.

BUT, according to PBS rules, if the format says hardcover, that is what must be sent..... or the data correctors need to fix that listing.

This help doc covers the various scenarios:

When I enter the ISBN on my paperback, the listing shows a hardcover!