Passing a variable from template into system

Hi all,

First of all I must say this is a great piece of software and you guys are a great community to back it up! I'm psyched to begin using this software on a lot of new projects.

So here is my dilemma.... hopefully an easy fix?

1) I have a variable, $custom_total, working properly in file in /catalog/view/theme/default/template/custom.php which displays properly on the cart page.
2) I am trying to utilize this $custom_total variable within the /system/library/cart.php file. How do I "call" this variable into that file?

Any help is appreciated!

Thank you guys,
opencart_hero

When I create a session and define a variable in /system/library/cart.php I can pass it to /catalog/view/theme/default/template/etc ... but not vic versa.

How can I do this vic versa?

Please someone.