How to store password on a user account to make certain content available?

I want individual accnt holders to be able to pay for specific downloads of their choosing. so I do not want large general categories like gold and silver accts.
I have a software package I would like to sell "upgrades" to.
but they cannot just download the "upgrades" they must do it thru the software package and that's why I would probably need a password type of system. The software would check the password file and allow the download.
How I explained that correctly.
Does you have plugins now available that could do such a thing?