To customize the "continue shopping" text on the Sell Media checkout page, paste this code into your active theme's functions.php or create your own plugin with your tweaks containing this code.

customize-sell-media-continue-shopping.php

<?php
/**
 * Plugin Name: Sell Media Continue Shopping Filter
 * Plugin URI: https://graphpaperpress.com/plugins/sell-media/
 * Description: Filter to Sell Media "Continue shopping" links
 * Version: 1.0.0
 * Author: Thad Allender
 * Author URI: https://graphpaperpress.com
 * Author Email: support@graphpaperpress.com
 * Text Domain: sell_media
 * Domain Path: languages
 * License: GPL2
 */
 
 /**
  * Change the http://example.com link below
  * to your custom continue shopping link.
  */
 define( 'CONTINUE_SHOPPING_LINK', 'http://example.com' );

/**
 * Change the empty cart message
 */
function my_sell_media_empty_continue_shopping(){
    echo 'Your shopping cart is empty. <a href="' . CONTINUE_SHOPPING_LINK . '">Continue shopping</a>';
}
add_filter( 'sell_media_continue_shopping', 'my_sell_media_empty_continue_shopping' );

/**
 * Change the continue shopping link if cart has products
 */
function my_sell_media_continue_shopping(){
    echo 'or <a href="' . CONTINUE_SHOPPING_LINK . '">continue shopping &raquo;</a>';
}
add_filter( 'sell_media_or_continue_shopping', 'my_sell_media_continue_shopping' );