It's been about 10 years since Harrison Schmitt, Apollo 17, conducted a paid signing at NovaSpace, and he hasn't responded to mail requests either.
For many years, Schmitt would answer his mail. Before he stopped, he would sign once a year, usually around the holiday season. In December, you would hear reports of a sudden flurry of through-the-mail successes. Looks like Harrison Schmitt is working his way through the stack from this year was the conventional wisdom.
In the past few years, prices for Harrison Schmitt signed photos have skyrocketed. You can expect to pay $400 and up for unpersonalized examples. Prime examples on NASA glossies or lithos with nice inscriptions can fetch $600 or more.
In my estimation, you have a new generation of collectors in the past decade that were not around when he was signing freely through the mail. This influx of collectors have to pay for signed items on the secondary market... something prior generations of space collectors did not have to do.
With this increase in price comes a higher risk of relatively skilled fakes. In the past several months, I have seen several examples of somewhat deceptive forgeries... the likes of which we have not seen previously.
As always, do your homework, caveat emptor and have fun collecting!