After tinkering obsessively, I have solved the problem.
As stated above, this is the failing shortcode entered in the post
[product product_sell_price=$765 product_description=20”x16” oil on stretched linen. Varnished for protection and ease of cleaning. Includes the frame as shown and is ready to hang.]
This is the shortcode that works perfectly:
[product product_sell_price=$765 product_description='20”x16” oil on stretched linen.<br>Varnished for protection and ease of cleaning.<br>Includes the frame as shown and is ready to hang.<br>']
Do note that the single quotes are vertical, non-curly single quotes and not standard curly single quotes. iPhone/iPad keyboards have key options for certain keys. This one is found by tapping the 123 key then the #+= key then holding down the single quote on that keyboard you will be given the option of ‘ ` ‘ and ’.
I hope this helps others as these types of problems sometimes take hours of head bashing to resolve 🙂