First of all I want to say that this is an ugly way of doing things. Most probably the ugliest. If you didn't want to add the company name in here I will. It's **interiorwebdesign.com**. Most probably the things sent to their server are the magento version, the extension version and the url of your website. You can check for sure by doing `base64_decode` on the value of the `sdata` parameter sent to them. I don't know what the answer should be if your licence is correct, but in case there is an error this is the response:

    checkout.o_p_c_l_s = '2';
    deljsphp();

Look in the code for a function called `deljsphp`. I think that disables your checkout.

**[EDIT]**  
Because I get mad when I see these types of extensions I decided to waste some time for investigating.  
Here is what I found.
The result of the request should be a link to a js file or some js code on their server. After decoding the js file from the extension I found this function:

    start: function () {
        this.s_code = $('opc_s_data').value;
        $('opc_s_data').remove();
        if (this.s_code == '') {
            this.opcdis();
            return false
        }
        addjsphp(this.s_code);
        this.getActionResults(0)
    },
so if the code 'license' is valid some js is added to the DOM through `addjsphp` that looks like this:

    function addjsphp(u) {
        var me = document.createElement('div');
        me.setAttribute('id', 'opcstataj');
        var p = 'html';
        var th1 = document.getElementsByTagName(p)[0];
        th1.appendChild(me);
        var th = document.getElementById('opcstataj');
        var s = document.createElement('script');
        s.setAttribute('language', 'javascript');
        s.setAttribute('type', 'text/javascript');
        s.setAttribute('src', u);
        th.appendChild(s)
    }

I don't know (yet) how the js from their server should look like, but here is what you can try. Not sure if it works. Edit the js file `skin\frontend\base\default\js\onepagecheckout.pack.js` and at the end of the file add this:

    function deljsphp(){
        return 1;
    }
This should override the original `deljsphp` function, that by the way looks like this:

    function deljsphp() {
        var th = $('opcstataj');
        if (th && th != null && th != undefined) th.remove()
    }
Basically it removes the `<script>` tag added from their server.

**Other info**
The code that is being eval'd in the the line you mentioned in the question is this:

> eval(gzinflate(base64_decode('jZAxb8MgEIXn+FfQCBl7qL1UHRLRDFGkLlWjOM0SVRHBV4NqwIKLqqrKfy84S9MuXeDgcd97BzWiI5xQVDrcPnSAT6IDi24HPmhni3KeUePaqyfP6+VPWV2Jy5Nv0Hl48X1UJ5OMhqhPow3/onE95xGXSteec2WFgXhQ51xIjEAuFcj3aaS2AkVsPIoA93cHsNK1UNAwGiZicQNmwM+CHprVZrfa7Nnjdrtu2GtJ8pz8ueWcM2dZuWAKcQhsNu4s0k6+TwOoULFZXWuL4LXzH3BsIejOVtKZ2lw+pXaD7LUEGyCVl8jVoIZFSHE5q8bY80y/FWlAIJwThhCQlb+NeidFr1zA/0K/AQ==')));

Going further...this is the code that actually gets evaluated:

    $mag = $this->getMagentoVersion(); 
    $mod = $this->getOPCVersion(); 
    $h = $this->getCurStoreUrl(); 
    $s = "mag={$mag}&mod={$mod}&hname={$h}&action=check"; 
    $data = base64_encode($s); 
    $hs = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS']==='on')?'https':'http'; 
    $url = $hs.'://interiorwebdesign.com/magento/opclicense/opcaction.php?sdata='.$data; 
    if($mode == 'test') 
        $url = $hs.'://localhost/opclicense/opcaction.php?sdata='.$data;

This confirms my initial suspicions on what is sent to their servers.