Good news!
This script was originally created for my own use to work with modernbill. That was originally its only purpose, of course it has flowered since then 
Here is an example of what I have in my "Available domains" template in the admin panel for a typical available domain:
Code:
<div>
<b>[[domain]].[[ext]]</b> - [[price]] (<a href='/client/order/orderwiz.php?submit_domain=register&domains[]=register|[[domain]]|[[ext]]&submit_package=domain&submit='>Order Now</a>)
</div>
As you can see from the example, my modernbill install is /client/ (basically, it is the normal install, but with /modernbill/ renamed to /client/). So simply modify this to whatever yours is and it should work just fine. Also note that the package is "domain", make sure you have enabled in MB to accept domain only orders (I am not sure if it is on or off by default). I think the "domain" package is built into modernbill's default installation.
However, please note that all this will do is auto-fill in the domain and extension into the MB order form. The customer STILL has to click through, MB STILL has to lookup the domain, and the customer STILL has to select length of registration and click next.
You can probably get around this (go search through that big MB manual), but I don't think it deters customers all that much.