PayPal makes several “pass-through” variables available which can be used to match payments with a customer ID, transaction ID, or any other pertinent information which you supply for tracking and identification purposes. The three primary pass-through variables are “item_number” (best used for customer or product ID), “invoice” (best used for a unique transaction ID; PayPal will enforce uniqueness on this variable in your account), and “custom” (best used for miscellaneous information that is not disclosed to the buyer). All three of these variables will display in the merchant’s email notifications, transaction details, downloadable log, and Instant Payment Notification (see below). “Invoice” and “item_number” are similarly disclosed to the buyer, but “custom” is not.
For example, when a subscriber returns to your “Successful Payment URL” after signing up for a subscription, you can dynamically display a page giving him immediate access to the subscription.
With Subscriptions Password Management, PayPal can generate unique usernames and passwords for your subscribers, enabling them to access “member only” content on your site.
After a customer signs up for a subscription, he will be given a unique username and password on the PayPal Successful Completion page. You will then activate the username/password pair on your website’s server through code you write.
Merchants using “Basic Authentication” with an Apache web server on Linux may use a sample Perl script developed by PayPal that provides complete Subscriptions management. The Perl script is easy to install and interacts with PayPal’s Instant Payment Notification interface to automatically activate and deactivate subscription accounts.
