[Plugin: eShop] Change Options Sizes Display

I’m using eshop and instead of displaying the size options in a dropdown select or radio buttons, I’d like to display them with buttons that I will customize so that it will look something like http://www.gap.com/browse/product.do?cid=17082&vid=1&pid=834098 (see size selection).

I’ve tried to modify the eshop-add-cart.php file but without success. I’ve achieved to display buttons but when I try to add to cart, it doesn’t work.

If anyone could help me, I’d be very grateful.
Thanks

http://wordpress.org/extend/plugins/eshop/