Approved affiliates for a product can offer a bonus for customers who buy through their link. You can attach an extra download or the link to a special page that you wish to include in your promotions.
Simply go to your affiliate's tab and click the "Add Bonus" button. That will bring up a screen with 4 form fields on it. The first field is a dropdown list where you can choose which product this bonus is for. In the next field, enter the name of your bonus, whatever you wish to call it. The 3rd and 4th fields go together, in that you can either enter the URL of the page where your bonus is or you can upload your bonus as a file. Click "Save Bonus".
Once you add a bonus, whenever a customer buys the product through your link, they will receive access to your bonus automatically, with no more input from you. The bonus is delivered right under their purchase in their customer account.