For a few versions now, SketchUp has had it's Extension Warehouse built right into SketchUp. Think of this as a built in app store. From there you are able to download and install extensions right into SketchUp.
No more do you have to worry about finding the plugins folder, or wondering if your extension has been updated. This new slick process of extension installation has been great for a while now.
Developers have been looking for a way to sell paid extensions for a while. Being a developer I can tell you that processing payments is a real headache. SketchUp has taken that headache away by doing all that right in the store.
So now if you want a paid extension you can go right to Extension Warehouse in SketchUp pay for it, and download it. All you need to do is go to "Window>Extension Warehouse"
I hope that we'll start to see an influx of some really pro plugins like this one.