The system probably sent her a Welcome email when she completed her login to PayPal and returned to your store the first time. That email should have her login password in the bottom of it, so she can login to her account on your store (using her PayPal email address and the password in that email), and her cart contents will be restored.

A loop-back like you describe is usually caused by something odd in the billing or shipping address or by some rejected response from PayPal.

Viewing the PayPal logs could be helpful, to see if any error response came back from them. http://www.zen-cart.com/showthread.p...Logging-PayPal

Also, any automated error-message emails from the store to the ZC storeowner could be revealing, if any.

And the fix mentioned in this post would be appropriate if you're using v1.5.0 and virtual products: http://www.zen-cart.com/showthread.php?197182-Known-Bugs-(and-fixes)-with-v1-5-0&p=1139882#post1139882