[zhodge]

I don't recall ever having any issues myself when donating for a Humble Bundle, but I frequently enter the wrong characters on other sites' CAPTCHAs, so I totally understand your frustration. Actually, I think it's because I'm always (just barely) beating the average: "Orders below the average must authenticate with reCAPTCHA to help cut down on costly abuse." Is this the CAPTCHA you're referring to?

[nilved]

No, I paid above the average. I was given the CAPTCHA when trying to log in to my existing account.