They do make very good plugins though... and the amount of free ones you get more than makes up for the quirky installer.
For plugins I've bought, I've found it much easier to do all the registration/redeeming etc on their website.
You can then download a single license file with all your licenses in it, and stick it in the one place.
The first time you've bought something, you just point it at the location of your license file when you fire up the plugin.
When you buy another, just download a new "bundled" license file and overwrite your old one. Your plugins will automatically pick it up.
My only real criticism of the Melda stuff is their GUI - you either like their standardised approach or you don't.
One thing is for sure though: they sound a lot more expensive than they look.