Zynga’s PetVille, a Facebook pet-caring game, is showing a streamlined direct payments interface in its offer wall. The new interface shows users the choice to buy Facebook’s Credits virtual currency before any other option.

The goal is to get users buying the in-game virtual currency, called PetCash, via their Credits accounts versus other methods. Although Zynga has been experimenting with Credits for months, this is the most prominent use of it yet as a payment option in a major game.

PetVille, with a flat 21.4 million monthly active users and a slightly falling 4.46 million daily actives according to AppData, now shows you Credits as the only option. Paypal and credit cards are available through Credits purchases only, but no longer otherwise available. Also, a new way to purchase Credits is in the mix: Japanese credit card conglomerate JCB International.

The largest Zynga game, FarmVille, has tested Credits as the first option, but it currently shows it third behind credit card and Paypal payment methods. So, we assume that PetVille is an early test of this new implementation, and we may see it elsewhere in Zynga games shortly.

All alternative payment and advertising offers otherwise remain on the wall, including a variety of mobile and pre-paid card vendors as well as SocialVibe’s offers. Facebook has incorporated mobile payments via Zong and it is experimenting with running offers so users can earn Credits. But for now, at least, it doesn’t seem to be going after the other options.

Eventually, though, expect Facebook to integrate every payment option into Credits. The company basically said this was the plan during its f8 developer conference last week. Facebook founder Mark Zuckerberg also has said that “there’s just going to be one currency that people use” in apps, meaning that the other options might be forced entirely in to Credits rather than being available separately as they are now.

All this remains to be seen. It is not clear how Facebook is working with Zynga and other partners to run tests. We don’t know what its specific process is for determining how Credits can or can’t be deployed versus other payment options in third parties. What we do know is that the company thinks Credits will help developers monetize, and that it is taking a 30% cut of all payments transactions itself. We’ll be covering as the service evolves.