Thank you raincoaster, I did follow that link and it led to a list of companies that provide digital download services.
Kathrynwp then said "You may be able to use one of those services in conjunction with WordPress.com as long as the code they provide for you to add to your site meets our requirements." and then she said "Just let us know if you have further questions."
My thought was that WordPress has likely had the experiance of dealing with one or more of these services and likely would know (based on that experiance) which ones provide code that meets their requirements.
I didn't mean to suggest that wordpress should do the research for me, but rather that (based on past experiance), the services that do and the services that don't are already known to wordpress and kathrynwp might have that information available to her (and us all) at wordpress. Thanks again raincoaster and kathrynwp for your help it is much appreciated.