After having success with your last suggestion, I started adding more fields to the import string. Here is the last one that I tried:

"product_type","sku","alias","title","introtext","up_sells","cross_sells","taxprofile_id","thumb_image","main_image","catid","price","shipping"
"simple","Z115"s,"z115s","Title","","J001","J002","","","","14","",""

It imports successfully and I can see the product on the front end. But when I go into the admin page to change or add anything I get this message:

"Content Encoding Error
The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression.
Please contact the website owners to inform them of this problem."

I started with data in each of the fields above, and then tried to troubleshoot by deleting the text (one at a time) until I think it is one of the fields themselves creating the error even if empty. I'm saving the import string as Unix, UTF-8 format. It errors if I fill that out or not in the app. Any ideas on what could be causing this would also helpful, plus perhaps it helps you as you update the app.

"product_type","sku","alias","title","introtext","up_sells","cross_sells","taxprofile_id","thumb_image","main_image","catid","price","shipping"
"simple","Z12","z12","Title of book","The intro text","z111","z111","1","images/B/B110.jpg","images/B/B110_thumb.jpg","14","12.95","1"

This string creates a product that is viewable on the front end of joomla5. I can put it into a basket, the images are correct, etc. But when I go to the backend to edit it I get the error message and cannot edit it. I'm using a Mac but the server is Unix. The string above is saved as a txt file from BBedit with UTF-8 format, unix returns. The odd thing is that it works with no error and appears on the user front end, but I cannot edit it to add anything or change price, etc.

Finally have most of my "simple" products moved over from my old joomla3 database to the joomla5 database. One suggestion I have is when getting a "duplicate sku" error message, it would really be handy if it displayed the sku that was causing the error. I think these errors occur when the item has been deleted from the original database but apparently have only been turned off instead of being purged. I ended up importing a category at a time instead of a batch, and on some I had to import 10 at a time until I found the offending sku and deleted it from the csv file.