Cart information is stored in session variables.
When A session is started, a cookie is created on the client computer to track the session.
cookies are based on domains.
the problem you are having is that you are crossing domain between the checkout page and eCart page when paypal returns you.
When you are at:
you are are on one domain:
paypal is returning you to:
which is a differant domain:
there is no www.
as far as session cookies are concerned, these are two separate domains, with 2 separate sessions.
on the confirm page, set the return URL to use the domain.