Commerce extra panes is a commerce contributed module that allows you to expose nodes in the checkout process of your site. Through an administrative interface, you can select the nodes you want to be in the checkout and then you'll have them available to select, drag and drop within the default checkout interface of Drupal Commerce. It also provides a submodule for Terms of Service that presents a checkbox so the user has to accept the purchase terms before proceeding with the next step of the purchase.

The module provides a view mode for the content types so you can select which fields of the node are visible in the checkout pane and it also supports internationalization.