@yvs I talked to the creator of my theme as well as my hosting company and both are clueless; it has to be something in Cart66's script.
But... until someone gets back to us (lol); you can try this... It didn't work for me, but maybe you'll get lucky.
Change the permalinks to default.. apply/save.. then swap back to Post Name... apply/save... Hey, it may work for you. I will tell you I think it does have to do something with the permalinks.
I've also tried disabling/re-enabling the add-on... As well as disabling all of my other add ons.. no joy. Even my older receipts won't show up; so there has to be some scripting error.. or more than likey; a permalinks issue.