Ok,
Im looking for a billing system...
I think im going to purchase Whois.Cart
Only one thing on my mind.....
How easy is it to integrate with your website?
Can you give an time estimate on how long it will take?
www.aquinox.co.uk, will I need a complete rearrange?

If you have a site using Whois.Cart, let me know :)

Best place is the forum topic dedicated to customising/skinning on their site.

Their main site also has a featured sites so you can see integration:

http://whoiscart.net/customers.php

Basically, it uses a 'themes' templating approach whereby you can change the look and feel for all the cart pages.

Time: depends on your experience of having done anything like this before. Is so then a day would be enough, otherwise ... who knows :)