I just tested again and it did the same thing for me. After logging in at paypal it brought me back to an empty cart.
URL in PM and my credentials.
And then I tested with your credentials using this same test product for $1
https://customersite.com/products_results.php?S_ProductName=&S_ProductPrice=1&S_ProductPrice2=1&Search=Search
Goes to paypal
https://www.paypal.com/webscr?cmd=_express-checkout&token=EC-moreLettersAndNumbers
Came back to confirm page and then receipt. So it is working with your credentials which are in a different group than mine. Would that make a difference?
Using my credentials it just comes right back to cart instead of confirm.