Is there a way to offer plugins al-a-carte?

I thought that by installing pretty plugins I would be able to offer plugins as al-a-carte. In other words, a client may upgrade to premium (I use pro-sites) and be paying $20 a month (example). upgrading to pro may offer him 10 more features at $50 a month, but he doesnt need and wont use all of those features. So I would like to offer him the ability to pick a feature, or two, and pay - say, $5 per month more for each feature (plugin) added.
Thought that was what i was getting with pretty plugins, but it only puts a pretty face on the plugin area. The client still has to do full package upgrades to get certain plugins.
any suggestions?