You know, I was surprised to see you and FL both mention Amazon, as I've never had a problem purchasing anything at Amazon, though I have to say I've only purchased one "expensive" item from them in quite a few years, and I wound up having to return it, and now have a "gift card" at their site for the full amount of the returned item. Now-a-days, I only purchase Kindle eBooks, most of which are free.I know it is more hassle but if you wanted something from Amazon you could set up a prepaid debit card.
As far as Renderosity goes, I'm one of those who's switched to using my PayPal account back in November, as it's easy enough for me to add funds from a small checking account I keep for just that purpose.