incomplete downloads onto Safari from 'Your music' on Amazon

My music purchases from Amazon are classical CDs, and a number of them have a free MP3 'autorip' version.

I chose to download onto my browser, Safari. When the download was completed, a 'zip' file appeared in my Downloads folder. I would then double-click it to convert it into an MP3 file, which I would then drag-and-drop onto my 'albums' window in iTunes.

I noticed, however, that some of the box sets with multiple CDs were in fact only part downloaded. I tried repeating the downloads several times, but with no success.

I have read very critical reviews of the Amazon Music App, and would prefer not to have to resort to it if I can possibly avoid it.

Any troubleshooting suggestions very welcome.