We're using PayPal Standard and it works 100% sometimes. Most of the time the last step, returning to the website returns the error in the screen shot at https://www.caladiumsdirect.com/210323_PayPal.jpg
Any idea what I'm doing wrong?

This has come up here on the forum lately, and there is a fix in the latest update package.
You probably want to subscribe to that forum so you get emails when new updates are released.

https://www.ecommercetemplates.com/support/forum.asp?FORUM_ID=20

David