Clearly, this means that the game is in imminent danger and is going to shut down within the hour, if you go by the forum thread on the matter. Or, as people point out on Reddit and elsewhere in that same thread, it just means… you can’t buy that set of digital goods through Amazon any longer.
A number of speculative reasons have been put forth for the change: A new package being offered for a new expansion, a change in how profits are being shared, a desire for people to move through the official store fronts, or just a routine set of business decisions. There’s been no official statement on this change at this time, but there’s not much reason to think the sky is falling just yet.
Source: Official Forums, Reddit; thanks to MikeyMoo for the tip!
Update: It appears Amazon has resumed the sale of these, minus 180-day subs, so you can stop panicking!