Even if Amazon would claim the money you owe in realtime, they probably wouldn't shut you down if your prepaid hits zero.
I use a prepaid for my Kindle purchases and Amazon processes the order even if the payment gets rejected, following up with increasingly insistent reminder emails.
Yeah, that's what I thought. I wonder why Amazon doesn't support some form of prepaid service. I feel that it's really a deal-braker for me for buying s3, these kind of horror stories scare me away.