Adding text before and after cart contents block in Drupal 7 commerce
The easiest way to do this, for me, is by code, as checkout pages are normal Drupal forms and as such, we can use hook_form_alter() to do whatever we need. In this case, you can use 

(yourmoduleorthemename)_form_commerce_checkout_form_checkout_alter (&amp;$form, &amp;$form_state)


And inside it, you can use Form API(https://api.drupal.org/api/drupal/developer%21topics%21forms_api_reference.html/7#markup-attribute) to change the way you want.

You can also create a block and put on the "checkout/%/*", but it generates a lot of extra markup for almost nothing.

I believe you can solve your problem with one of these, but let me know if you need more details.