[totaltec]

This is defined in your css:

.flc-checkout-container {
    clear: both;
    padding-bottom: 30px;
    vertical-align: top;
    width: 100%;
}

But apparently not referenced in your tpl. I think Steve's solution with an if statement would be the easiest fix.
Replace: <div class="center-main">
With: <div class="center-main{if $main="cart"} flc-checkout-container{/if}">

Then adjust your css for .flc-checkout-container to suit your design.

Obviously you would need to test this throughout the checkout process and with different browsers, firebug shows the fix working in Moz on the first page of the cart. but I think you need to add a fixed width in order to suit your rounded corners styles.