I learned the hard way with Yahoo, I put in $1000 to play with and bid on "free games" for our gaming site, the top bid was only .20 or something, I had no curbs in place, started the campaign 8pm in the evening, fell asleep, woke up to the funds gone, and when I tried to add more, said I needed 3K.
You can phone them and explain you're new and you didnt know, theyll waive the forced payment ONCE I believe (the guy told me this), they probably update your profile with something like "ANOTHER SUCKER GIVEN A SECOND CHANCE" so you cant beg them twice...