Last week I received a request for an old paperback (1975) titled Richard Hittleman's 28 Day Yoga Program [of something very similar]. A condition was that the book sent match the ISBN shown.

Holy Cow, my book didn't even have the same title. It was Yoga for Physical Fitness by Richard Hittleman. I can't exactly figure how this happened, as the book I have in my hands right now doesn't even have an ISBN, was posted as a book without an ISBN. I sent  a PM to the person describing the book I have and asked ship or no ship. They said no ship. So I just let the book time out without shipping it. Is there some way I can/should send PBS to inform them why I did as I did?

Actually, if your PM was sent from inside the order, it will be attached to the order archive.   PBS has access to that.  You are covered. 

That said, how did the title get edited if you posted without an ISBN?  That's fishy.
 

It's hard to know exactly what happened without having more details, and the book "Yoga for Physical Fitness" is no longer on your bookshelf.

One possibility is that the requestor may have been responding to an email notice from PBS that a similar book as one on their wish list has been posted. Here is an excerpt from a "similar-item" email that I received recently.

"This is a quick email to notify you that a book similar to one that you are wishing for, 'Book Title' by Author Name, has been listed in the system by a club member. ....

This is NOT an exact match to a book on your Wish List (it is a different ISBN, and maybe also a different binding type), so this copy is NOT being held exclusively for you the way that a book from your Wish List would be. This book is being offered on a first-come, first-served basis to all of our members. ....

A second possibility is that the member was browsing for yoga books by Richard Hittleman, saw your book posted, and requested it with the condition that the book match the ISBN of the WL book even though it had a different and was assigned a 6-digit number instead of having a true ISBN.

Does the listing still match exactly what you entered it as? Or has that been changed?

If your listing is still OK, then they may have tried ordering it thinking it might be what they wanted and sent the conditions just in case it wasn't (as the case turned out to be). You shouldn't need to do anything else in that case, you did exactly what you needed to do. Make sure to re-post your book since allowing it to time out removed it from your Bookshelf. Just check to see if that member is now on a Wish List for it before posting.

If the listing has changed, send in Feedback and ask them why your listing has been changed when it is supposed to be for your copy and your copy only. You'll want to get the listing corrected before re-posting the book.

So I just let the book time out without shipping it

This was a weird situation, but try not to let orders time out. It makes your account look inactive and can prevent the order from rolling over if there is another copy posted. In this case, I would have sent a message using the 'contact us' link at the bottom of the page to explain. You should still be able to see the listing on your Transaction Archive and I would include the info from it if you do send them a message now. I think you should. Like everyone else, I think it is odd that you posted it without an ISBN and the entry was changed.

A condition was that the book sent match the ISBN shown.

This is a standard rule for all books, but was there also a Requestor Condition attached to the order that repeated this? If so, the best way would be to use the button that says "Does Not Meet" the RC. When you pick that, you can explain what is happening and if the requestor did something odd, they may figure it out.