How do I add custom text to the shopping cart page?

There is a fragment called RETAIL_STORE_SHOPPING_CART_UNDER_ITEMS. You can create it in Orders > Setup. Click on the Fragments tab, filter by the word Under, and you will see it in the list.  Click the dropdown and select Customize. Then click Save. That will create the new fragment. 

To edit the new fragment, go to Website > Fragments and find the fragment called Shopping Cart Under Items.

To use a rich text editor instead of HTML code, you can click the formatting button in the lower right corner of Content box. The button looks like this: