I believe this address
fls-na.amazon.com, is used for authenticating apps “purchased” (paid or free) from the Amazon App store.
With it blocked the Plex Amazon App puts me in “Trial Mode”, and is virtually unusable, but after whitelisting the address it validates my purchase just fine.
I would guess that it shows up in so many lists due to some Apps that use bad/malicious ads, but also authenticate purchases through the Amazon App store. Given that it’s an Amazon domain, I’m pretty sure it’s (mostly) safe from malware, and likely not the direct source of ads.
Other similar, and unblocked, Amazon addresses are:
After some googling it seems the ‘mas’ is the “Mobile Associates API”, where ‘ext’ is for in-app purchases (like Movies in Amazon Video), ‘sdk’ seems related to the API itself. The ‘ssr’ seems to be related to the App Store itself, but I’m not sure how.