Hi,
I don't know why but on that page you should have :
<section class="span6" id="main">
like you have on the other pages while you actually have:
<section class="span12" id="main">
If you change it with span6 with firebug, you will see that it displays fine without your CSS code modification.
This HTML is output by your template so I don't know why it's doing that only on that page though.
In any case, if you want to change the CSS .row-fluid .span12 { width:100%;} you can edit the frontend CSS file of HikaShop that you can edit via the Display tab of the configuration.