OK, send a message through skype is you still need help.
For the issue you are reporting where some users are getting a failure on directing back from paypal, this is probably a case of domain switching.
when items are added to the cart, they are stored in a session.
If the user browses to your site using the domain name without www in front and starts adding items:
http://mydomain.com/shop.php
and is then redirected to the pp_confirm page with www in front of the address:
http://www.mydomain.com/pp_confirm.php
this is considered 2 separate domains and 2 separate sessions. to prevent this, you can use htaccess to force the address to always point to www:
http://www.htaccessbasics.com/force-www-nonwww-domain/