I foolishly failed to notice the full name of the program, and was peeved at the idea of spending extra for features I probably didn't need.
Since I don't like to do irrevocable things, I marked the pop-up "Remind me in 30 days".
I'd previously been able to attach my scans to email, but I couldn't get a scan I made after the pop-up happened to attach to email.
So I contacted the ebay vendor I'd gotten the scanner from, and they said that there was no time-limited software sent with the scanner, quite reasonably, that they needed more information or a screen shot.
So I made another try at attaching one of the newer images, and this time it worked.
I can easily believe that I made some error earlier when I was trying to attach an image, but I don't believe I hallucinated the pop-up.
How can I find the mysterious pop-up program, or do I just need to wait for a month?