Hi,
Setting any session variable require just a basic command($_SESSION['myvar'] = '1'), in this case 'myvar' will be the zipcode session variable. we will have to set the value of this session variable at product page and cart page. I have been working on magento for last 3 year. most of the project that i have done are on magento. If you would like to discuss more about my understanding of the project, just send me a message on freelancer and i will be online in no time.
Best Regards
Sumit