That’s what I’m going through…I’m pretty sure it does apply to Amazon Music based on my tests.
The thing is, though...Datacap's are usually not "You have reached 1 GB, we will turn off your data, until next month"
Most i know of is more like "You have reached 1 GB, from now on, your data is capped at XX KB"
You would still be able to get live traffic, as this is not a lot of data, but streaming, would be impossible.
I obviously do not KNOW how they have set it up with this.
Another thing that should not be part of the cap is the OTA firmware.
I just updated my android, it was a 1,6 Gbyte update. If the radio got a similar update, it would not be able to download it in one month as the update would be to large for the dataplan. For this reason, OTA's will probably also be omitted from the dataplan "counter".