try adding the following to your css
#shoppingCartDefault {
background-color:#FFFFFF;
border:1px solid #4A5D2A;
margin:10px;
padding:10px 5px 50px;
}
That should change the bottom padding on the checkout to include the buttons.

It's similar to the .centercolumn call but, if you change that one, it will change all instances of the center column.