When you click the link to download the ZIP file for a plugin you've purchased, you may find that it appears to be a folder of files instead of one ZIP file.
This is usually because Safari tries to be 'helpful' and unzips the ZIP immediately upon downloading. This isn't what you wanted - you need the ZIP intact.
You should normally still be able to find the original ZIP if you go to your Downloads folder, or you could try 'Compressing' the folder to recreate the ZIP for yourself.
Alternatively, you can change this behavior in Safari. Go to Preferences > General, and then uncheck the option "Open 'safe' files after downloading".
Go back to the original link and try to download the ZIP file again.