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It is a feature packed full site program with everything needed to fully or partially automate this type of site and comes with paypal billing module built into it. All aspects of this type site are covered in the program including a lost password/username retrieval tool.
Price: $189.95

You can automate most online payments, easily and affordably using an automated form script for Paypal. The script collects information, calculates totals, sends e-mail confirmation to the webmaster, and redirects to the Paypal order form with all the necessary information. Our script also ensures that noone gets access to the payment details inside the link or form tags, since they are placed in a data file on the server.
Price: $99.00 + customization fee

This script will integrate into any existing website by simply adding buy now buttons to your products. Handles complex issues such as multiple products with multiple variables and accounts for sales tax. Easily integrate with PayPal to process credit card transactions without a merchant account and the hefty fees associated with it. Software can be used with any on-line credit card company.
Price: $119.00

The PayPal shopping cart program is a CGI program written in a concert of Perl 5 and Javascript. This program was designed to work directly with PayPal. To use this program you must have an account setup with PayPal. The cost to have an account with them is for a low fee of just 2.9% plus $.30 on incoming payments.
Price: $75.00

PP Download for PayPal IPN is designed primarily for small-to-medium sized and home-based merchants who rely on PayPal for payment. It is secure and is incredibly easy to set up and use. PP Download for PayPal IPN might just be what you're looking for. Uses IPN system coupled with easy Paypal web payment buttons. Can work together with your affiliates tracking method.
Price: $35/$55

QuickDownload is a simple script to purchase and download files through PayPal. The application is written in Perl and uses PayPal IPN (Instant Payment Notification) feature to implement a simple and easy means to sell electronic files on the Internet.
Price: $10.00

Administration Panel, Affiliate/Reseller Area, Anti-Fraud Cart System, EBAY Auction Compatibility, Unlimited Authorized Accounts, Thank You Page Receipt Generation, Detailed IPN Logging, Detailed Statistics,Test Mode Simulations, Unlimited File Handling, Optional File Uploading, Leech Protection System, Authenticode Length Settings, Automated Link Generation, Provisional Link Generation, Link Download Limitations Per File, Link Download Expirations Per File and many more features...
Price: $49.95