For the past 2 years, I have been doing a good bit of work with WHMCS, developing customized modules, integrating design with websites, and a variety of other misc. services.
A perfect example of what I do can be found on my own website (http://www.linux-tech.net/) and client area (https://secure.linux-tech.net), as far as integration.

What's included with a whmcs integration?
WHMCS will be 'skinned' to look exactly like your main website.
Your main page can be edited to reflect ordering URLS.

Forms possible (depending on your website and business):
Login Form
Domain Check / Ordering

Custom Modules also available. I can very easily work with WHMCS to do a number of things. Please Contact Me (http://www.linux-tech.net/contact.php) with questions there.

Until March 1st, I will be offering WHMCS integrations as follows. Please contact me (http://www.linux-tech.net/contact.php) to order them:
Tables/Frames based webpages: $40
CSS based webpages: $25
All of the above (forms, skinning, editing) will be done at that price

what kind of modules can you do?

If it can be done using PHP, it can (probably) be done in WHMCS. There are, of course, limitations, but it's not that hard to get a custom module done for WHMCS, all you have to know is how to do it.

If you're looking for custom additions to this, the best thing to do is open up a support ticket, as WHT is not a support forum. I'll gladly help you out however I can there :)

Would the CSS option also be for CSS/HTML? Just wanted to know, since I've got WHMCS on mine, but suck totally at doing integration. :/

Hello,
CSS does in fact refer to html/css as long as there are no tables involved. Tables take a lot more work to integrate into the WHMCS setup than css does, which is why the price for them is a bit higher.