Ubercart external file download
Just a couple of ideas:


Alter the email template to include the external link to the purchased files.
Create a tab on the user profile with the links of of the files on the external site.
Create a module to allow the default "File download" module on Ubercart to link to external files and not only to upload ones.


Other readings:

Creating an E-Commerce Site Using Drupal &amp; Ubercart

Selling Digital Goods (Mp3's, Photos, Movies, etc)