I really love the feeling of safety that I get from chosing the max amount, being sure that I won't be overcharged.
So unfortunately if you get points I'm guessing you're from North America and probably won't be able to apply.
I haven't searched but there might be such services available to you in a nearer bank.
Edit: As a meta-sidenote, it was very difficult for me to write this comment without feeling like it would come across as corporate shilling. I have no connection to Citi and nothing to gain from recommending their card, but there's no way for anybody else to know that, and the issue of selling old accounts is a real one (though don't know about HN specifically). I wonder what the best solutions are to this sort of issue.
When I order a 20€ item, I generate a credit card number valid for a single use of 20€. If the merchant tries to charge 19.99€ or 21€ or any different amount it will be rejected (and the merchant may have to pay penalty for payment being rejected).