There are some restrictions, which work out to basically one per customer (account), only those sold by Amazon (no third party sellers or used), and only until the number they've set aside for the promotion sell out. Although only the base models are shown on the deals page, the wording says on any model, so the $50 discount should work on the ones with more than the base RAM configurations, as well.
Limit one discount per customer. U.S. customers only. Discount valid on all versions of Kindle Fire HD 8.9” sold and shipped by Amazon. It does not apply to the same products sold by other sellers. Amazon reserves the right to cancel the promotion at any time. Promo codes may not be combined with other offers. If you return items purchased using a promo code, we will subtract the value of the discount from your return credit. Offer not valid with 1-Click ordering. Void where prohibited,