I tried to buy it but after the 7 step check out process it complained "ERROR NOT AUTHORIZED" for my credit card.

I have a copy lingering in my shelf for a couple of years now and just started a private project to implement the algorithms in Rust the other week...

It looks like they disabled their webshop:

  We are closed for business.
  New orders are no longer accepted.

This was a huge shock to me too, a few weeks ago. I emailed them, and Perry replied:

Well, we did not exactly plan to have this happen either although at 82 and after 47 years in the business we knew that our time was limited. What triggered this was that our custom written software, developed over 20 years started to fail and our developer abandoned the project. We knew that switching over to another package was a possibility but that it would take the better part of a year to accomplish and that was not appealing.

However, over the years we have received inquiries about selling the business. We have a fully qualified buyer who is now considering purchase. The business was profitable and had no debt and is a natural fit for this potential purchaser. These things take time but our books probably will be back on the market in 2 to 3 months.