Are you sure, other posters have claimed that their Amazon sales are much larger than their subscription / paizo site sales. I don't know how anyone without inside information would really know, but I don't think it is safe to assume one way or the other.
I don't have any links, but I do recall reading comments by Paizo insiders that their subscriptions account for something like half of their revenue. That was the case when they published Dragon and Dungeon magazines (obviously - all magazines earn the bulk of their money from subscriptions), and it carried through when they started the Pathfinder subscriptions. I also recall reading that core books don't make up all that much of their revenue, that they get more from their adventure paths. This may sound odd, until you consider (and I definitely recall James Jacobs making this comment) that half the people who buy adventure paths aren't actively playing an RPG, but buy them as reading material.
There's a reason so many entertainment providers are going to a subscription model. It's predictable, steady revenue. At $25 a month for the AP subscription alone, a subscriber will spend $300 a year. That's the price of six hardcover books. Nobody buys six hardcover books a year for the same game, on Amazon or anywhere else. Even if Paizo have 5,000 subscribers for the APs, that's $1.5 mil in annual revenue right there. And that doesn't include the subscriptions for campaign setting books, map packs, etc.
Paizo is a different company from WotC, with a different product suite and a different business model. It's a mistake to judge its health by Amazon sales of core books.