Try this, go to the advanced search page, input the ISBN you listed. Click the box to show large print only. If that ISBN comes up, your edit was correct. It may just have not come up in the queue yet. If it doesn't come up, then you are probably better to go with your second choice to post as a non-ISBN because the system does not recognize it as such. Thanks for being courteous to those that care whether they get LP or not.

Ya know... I was just thinking about this... seems like as good a place as any to mention it!  I listed some Harlequin title a while back and got that pop up window, which I just clicked through without thinking about it.  Later, I realized the book is only ALMOST a large-print. 

The listing isn't wrong, and I'm not wrong, it's just one of those newer "Larger Print" - bigger, but not big, I suppose.  All I could think was that Harlequin really is out to complicate things, aren't they?  :-O