I agree with the above, but disagree that this makes micropayments for online content a futile venture. There are a couple ways that come to mind to solve the mental fatigue issue:
1. Automate all the payments
2. Automate all the payments w/ a cap
3. Automate payments from whitelisted domains
4. Don't automate payments, but ask to whitelist new domains (similar to: "remember me").
These could all just be user preferences, just as described in the post.