Add extra html to ubercart cart and checkout pages
You can achieve this in following ways,
1) define a template file:- in your template.php, define a template file for checkout page

/**
* Implements hook_theme().
*/
function themename_theme($existing, $type, $theme, $path) {
  $items = array();

  $items['uc_cart_checkout_form'] = array(
  'render element' =&gt; 'form',
'template' =&gt; 'uc-cart-checkout-form',
  'path' =&gt; $path . '/templates/',//where the tpl files are located
   ); 

   return $items;
 }


Then in your templates folder create a file uc-cart-checkout-form.tpl.php, here you can render your form with additional htmls

2) Override the theme

Check this instruction on Ubercart How to Theme the Checkout pane in Ubercart 3, by following this you can include your custom html.

Similarly the above methods are applicable to cart page also