I got an email from PBS saying "A club member has requested your book Book Name"  If you can mail this book, then you will receive 1 Book Credit etc etc.

But when I look at my Account page, that book is on the usual 48 Hour Wish List Hold,

Shouldn't the email have been sent AFTER the member confirms they want it?

Check your transaction archive to see if there is a canceled transaction for the book.  It could be a case where the 1st person requested it, changed their mind, canceled it and now it is on WL hold by a 2nd person.

I don't see anything in the transaction archive.  In fact, when I posted the book, I immediately saw it on the Account page as a WL hold.  I clicked the PM to see the member's name; clicking the PM button now, I see that it's still the same name as it was when I first posted the book.