It appears you're using the FatFreeCart.com service, rather than E-junkie Cart. Please bear in mind that we provide FatFreeCart.com on a strictly do-it-yourself basis with no support whatsoever (that's a major reason it's free :^), so you're expected to figure it out on your own and troubleshoot any issues you may have with that. If you decide to upgrade to an E-junkie cart subscription (aka "FatFreeCart Pro"), that does of course include support. That said, I can try to answer your inquiry this time.
That border appears to be coming from your site's CSS; in http://lite-sites.co.cc/templates/litesites/css/template.css you have this declaration:
border: 1px solid #999;
You can just remove that (which would remove the border from ALL form inputs in your site), or you can override it for all form-submission buttons on your site by adding this to your CSS file, below the section given above:
...or you can override it for just the one button itself by adding a style="border:none;" attribute to that button's input tag like so: