Thanks Eric, but it's not resolved. I did figure out WHY I'm getting the error- it's because I am using charges. I tested it various ways and it only works when I remove the charges from the cart object. I do need to use charges, and I can't be the only person using charges and PayPal. What I need to know is how to fix it.
I realized that what I originally thought about closing the browser each time doesn't work. The session is definitely being closed each time.
I still need an answer. Thanks