Thank you very much for your response and your answer! (If anyone ever has the time, it would be great to include this info on this page:
http://docs.getshopped.org/documentation/paypal-payments-standard/ )
Before I received your answer, I tried to include the Notification URL that I found here:
http://docs.getshopped.org/documentation/paypal-express-checkout/
which is:
http://mysite.com/products-page/transaction-results/?ipn_request=true
Can I leave that in place in our PayPal account, or is it better if I remove that?