Did your customers say they couldn't get to the shopping cart at all, or that orders were not going through after they entered their shipping info? Do you know if the customers who couldn't place an order were using IE? I tried the site in both IE and firefox, and was able to get to the shipping info page in both browsers (was afraid of going farther for fear of actually placing an order), but in IE, your layout is messed up. I can see the "Steel Couture" header and sidebar, but where there should be merchandise, there is a giant white space. I have to scroll all the way to the bottom of the page to see it.
I took a quick glance at your source and the first thing that comes to mind is that I would have used tables to position everything on the page. It's been a few years since I've done webpages, but if that's your only problem, I don't mind taking a crack at it for you. I don't know how to fix shopping cart code though, so if it's more complex than your customers just not realizing they have to scroll down, you're going to have to find someone more coding savvy than me.