This is because your ssl redirect doesn't include the www. in front of the url and you are starting with www. in the url when you start your session.
I think the solution is to add an .htaccess file with this code:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^www.plus-skateboards.com$ [NC]
RewriteRule ^(.*)$ http://plus-skateboards.com/$1 [R=301,L]
That way it will never use www. in the url so that when it redirects to the https:// version the cart will remain intact.