The good news was a sale of 3 large files for $42. The bad news was that after Paypal went through the cart disappeared! I had seen the user registration email and checked to see if there was a live cart - there was. But after the Paypal email came through, there was no cart and no purchase! Fortunately the customer was in NZ and rang up. We sent the files by email, I checked Paypal and the IPN message had been sent.
Just updated to WP 3.6 today, and everything had seemed OK... Any ideas?