Recommendations needed.

We want to get in touch with customers via live chat.

Requirements:

• chat window customisable to mingle with website theme (colors etc)
• preferably the window should be within webpage and not only pop-out/popup.
• ease of use by customer
• minimally intrusive
• should have triggers/Alerts to backend side. for ex: user is unable to fill-up signup form or something, we should be able to offer help to user and this chat window automatically shows to user.

What is the cost ?

UPDATE: After R&D we also narrowed down to comm100 and liveperson, and we will go with comm100. LP is best commerical soln. but comm100 is a good free soln. We go with comm100 as starting point.

But it gives out exceptions alerts sometimes in the dashboard. Can it be configured for chat invitations triggered on certain conditions. Currently only a timebased trigger is available. Any other major difference between these two ?