I just ordered a Synology DS214Play from Amazon and received a nice surprise this morning. I received a email that the purchase came with one year of Amazon Unlimited Cloud Storage. I can't find this promoted anywhere on Amazon so I don't know what specific NASs qualify. While there's no mention of Amazon Prime, I am a Prime member so there may be a link there. The Unlimited subscription is typically $60/yr. which I already consder a good value.
I already have the Unlimited storage, just recently converting from a free (a past Prime promotion) to paid plan. The free year was added on to the end of my current subscription.
I included the email below, which is the extent of the information that I have. If you're looking for a Synology NAS it may be worth buying from Amazon, although like I said, I have no idea who (other than the mention that it is US only) or what qualifies.