Sometimes you have to be persistent. Here are two alternatives (I favour the Express Lane approach).
For everything you need to know about accounts. Apple - Support - iTunes - Account & Billing. which at one point leads to ...
http://www.apple.com/support/itunes/contact/ - click on Express lane followed by iTunes > iTunes store.
But before you sit down to compose a complaint, do double check that the details about your account are as they should be.
I follow your advise. I sent email to apple support and they sent the steps that was mentioned in the support modul which I tried several times and always failed! Today I just try again to update my billing payment info with the same info just like before (I try to be persistent) anddddd it SAVED! So now i can buy the apps. I'm afraid that someday the error happen again and I hope the same techniques will help!