Best I can do for you is a picture I took of the display they had in the shop when we were on the Fantasy in May this year:
View attachment 702730
So, when I was looking at the necklaces in May, the CM in the shop told me that Tiffany had just shipped them the remaining stock from the old partnership, and that once it sold out, that was it. In theory anything they didn't sell prior to the Wish's launch was going to be shipped to the Wish once it started sailing, but she didn't expect they would last that long.
Now, I see you can preorder these on the Dream and the Fantasy still which makes me think that either she was misinformed, they had way more stock than she suggested/they've sold a lot more slowly, or Disney and Tiffany negotiated to keep making them after all. I too have looked at those eBay listings and I assume that they consider the necklaces retired because for a time at least they were not available, and with the onboard Tiffany shop gone, it is reasonable to assume they would no longer be produced.
I did not end up purchasing the Fantasy-specific postcard necklace, partially because it was not apparent to me that it was a postcard. I only saw what you see in the picture, which to me looked like just a big tag that had the Fantasy logo on it. I have no particular affinity for the Fantasy, so I opted to get the generic
DCL set instead. I only saw the cute postcard side in looking at images online after I got home, and honestly I kind of wish I'd gotten one when I could...but maybe they'll stick around now?
At any rate, I hope your wife loves it and you have a fabulous cruise!