Firstly, many thanks for an extremely useful plugin. My wife's ereader is giving problems so I dug out my old Kindle 4 and have managed to transfer 109 books in 33 collections with minimal effort.
However, there is one small improvement in the instructions that I might suggest. I managed to interpret "Create Collections on the Kindle from Calibre" to mean that I could work on the books shown in Calibre and that the books would then be transferred to the Kindle, sorted into Collections. It was only after keep getting error messages about books not being on the Kindle that I realised that what I needed to do was first transfer all the books en masse to the Kindle and then use the plugin to sort them into Collections.
One interpretation of this is that I would probably manage to misread the instructions on the side of a packet of cornflakes.
You might care to make things crystal clear for the benefit of people like myself.
However, again, congratulations on the plugin!