I had a problem where I got an error: 100 message in Origin and it wouldn't install the game. Followed this link: http://forums.reckoning.amalur.com/showthread.php?3685-Install-Amazon-Game-Files-in-Origin and all is well. Hope this helps!
I started the install with the Amazon download, then once it opened up in Origin with an entry for the game, I canceled it (with the Clear option), closed Origin, and created the Kingdoms of Amalur Reckoning folder where Origins places its games. Afterwards, you can unzip the Reckoning.zip file to that folder, start up Origins, and it should read Ready to Install. Click that, and it will sit there for a while, then you should be ready to go.
Where was this "clear option" located? I am having the worst time I've ever had doing a digital download. I did unzip the Amazon installation file to the Origins folder, but now the Origins program is telling me that I don't have enough room or some of the files are locked. I have 520 GB free and I have no idea which files they could mean.