Find this in your stylesheet:
Code:
#headerWrapper, #contentMainWrapper, #logoWrapper, #cartBoxListWrapper, #ezPageBoxList, #cartBoxListWrapper ul, #ezPageBoxList ul, #mainWrapper, #popupAdditionalImage, #popupImage {
margin: 0em;
padding: 0em;
width: 720px;
}
#cartBoxListWrapper and #cartBoxListWrapper ul are both being defined as 720px wide.
You need to separate those selectors into two lists, one with the width and one without. Decide which elements you really want to be that wide.
"em" is a measurement relating to font size. It essentially is a width equivalent to an "m" character in the particular font; for general Zen Cart purposes, the base font is 10px high and an em is 10px - this makes converting between em and px easy. In places where you have different size text, an em may be a different size also. This helps when making page layouts that allow users to display larger fonts for reading without destroying the layout - the whole layout will expand with the text.