How do I expand the shopping cart field?

Hi all I am not sure if anyone here can help as this is to do with OS Commerce, but no one hasn’t got the answer on there web page/forum so Iv come here see if anyone can help me.

I am having problems expanding the shopping cart field. As the customer adds items to the cart and if the customer wanted to browse around the shop then the items are still there. Now if the customer adds items into the cart and then goes onto other pages like for example “Home Page, Retail Concept, etc.” and then goes back to the shopping cart the items have vanished from the cart.
Also how would I change the shopping cart page from being “Index.php” to another name without messing up the other pages within the shopping cart page? As I tried that as well it didn’t seem to work.

I thought I had a solution by adding the following lines within the other pages to try to expand the shopping cart field but that didn’t work.

require('includes/application_top.php');

require(DIR_WS_INCLUDES . 'application_bottom.php');

So can someone help me please on a solution to this.

Thanks in advance for any help.