For next release 20.0 we upgraded our license key system to be able to generate keys for all parts individually. For the store we just added an "other" item as place holder. A few people asked over the last year for licenses to individual licenses, so we got this now.
Please notice the dependencies list. For example when you would purchase AVFoundation plugin, you also need Main, MacBase, MacCG, MacCocoa, MacControls and MacCF installed. Those plugins must be installed to build an application using a single AVFoundation class. If you use one of the classes from he other plugins, you may need the other plugin license, too. That is the reason we offer users interested in those plugins the whole package to avoid them later run into an issue with a missing license.
Nevertheless, if you can't afford Complete Set and you need one of the plugins, you are welcome to order a license. If you take the other item in the store, we'll ask you which one you want. And if you later prefer Complete Set within a few weeks, we may just refund the old part.