To change the container header text do the following:
- Navigate to the dnn -> host menu and choose module definitions
- Click the pencil next to the Smith.Buynow module
- Scroll down to the bottom of the screen and you will see all the pages in the cart
- Click the pencil next to the one you want to change the title on
- Type in the title you want in the "Title" field
- Click save
All the pages in the cart checkout will use the container you select for the module, there is not way to choose one container for the cart page and a different container for the product details page.