Okay, so does BundleStars not allow actual Amazon Payments, or...?

I actually was not aware that you could use Amazon Balance for Humble Bundle. I have some credit in Amazon from a gift card I got a while back, so it's not like it's credit from promos, but it wouldn't let me use that credit for Humble Bundle. I thought it was just for people that would rather not give their credit card info to Humble Bundle. Maybe the region matters?

I'm in the US, so I wouldn't imagine there'd be any restrictions, but BundleStars is a UK-based company and charges purchases in GBP, so who knows I guess.

Amazon payments is for use with credit cards, like PayPal. Amazon credit is only for Amazon store.

You cannot use Amazon Payments with credits or gift cards on stores outside of Amazon.com. That includes Mechanical Turk money. You'd need to transfer it to your account first and then charge by card/account to use it for bundles.

I can do exactly that with Humble Bundle - I've bought numerous bundles and a few things off the store with Amazon Payments credit directly. Not sure how that works, since Amazon explicitly states that AP funds cannot be used for digital downloads, but there it is - it does require that I use a debit card for backup, but always asks if I want to apply my AP balance towards the purchase first and my debit card's never been charged a penny.

Anyway, it appears I have my answer: no go with BundleStars.

Also, just to prove to anyone questioning the premise that I'm not crazy - including myself - here's a screenshot I just made of the Amazon Payments checkout process for The Vanishing of Ethan Carter:

http://i.imgur.com/9VwqSmf.png