I would think that the best web based chat solution for you, considering you are using Opencart is the one provided by Opencart themselves, available from http://www.helponclick.com/live-chat-software/integration/opencart.html
It looks very easy to add (the website claims it can be done in just 30 seconds) and appears to do all the things required of a live chat program.
It will work on any computer anywhere in the world, and there are no system requirements.
The prices are clearly shown on their website (as above, just click on 'plans and pricing') and the best part is that you can have a 14 day free trial so you really have nothing to lose.
Another good thing is that they do not take credit card details from you when you sign up for the trial, so there is no worries that you will be charged if for some reason you find it is not ideal.
If you do like it and decide to continue, you can earn significant discounts by paying quarterly or annually.
Cancellation is easy, and they don't bind you into a contract, though as previously stated, discounts are available if you choose to sign up for three months or more.
All you need to do is to set up an account and after that it is a simple copy and paste and away you go!
The chat system can be used on up to five different websites, and different departments can also be created.