Remove default Sell Media shortcode [sell_media_item] and replace with a custom version

sell-media-custom-item-shortcode.php

<?php
/**
 * Plugin Name: My Sell Media Shortcode
 * Plugin URI: http://graphpaperpress.com/plugins/sell-media/
 * Description: An example shortcode plugin for Sell Media.
 * Version: 1.0
 * Author: Graph Paper Press
 * Author URI: http://graphpaperpress.com
 * Author Email: support@graphpaperpress.com
 * Text Domain: sell_media
 * Domain Path: languages
 * License: GPL2
 *
 * Copyright 2015 GRAPH PAPER PRESS (email: support@graphpaperpress.com)
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License, version 2, as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License or license.txt for more details.
 *
 * @package SellMedia
 * @category Core
 * @author Thad Allender
 * @version 1.0
 */

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) { exit; }

/**
 * Remove core Sell Media shortcode [sell_media_item]
 * and replace with custom [sell_media_item] shortcode
 *
 * @since 1.0
 */
function my_sell_media_init() {
    remove_shortcode( 'sell_media_item' );
    add_shortcode( 'sell_media_item', 'my_sell_media_item_shortcode' );
}
add_action( 'init', 'my_sell_media_init' );

/**
 * Adds the new 'sell_media_item' short code to the editor. [sell_media_item]
 *
 * @since 1.0
 */
function my_sell_media_item_shortcode( $atts ) {

    extract( shortcode_atts( array(
        'style' => 'default',
        'color' => 'blue',
        'id' => '',
        'attachment' => '',
        'text' => 'BUY',
        'size' => 'large',
        'align' => 'center'
        ), $atts )
    );


    $html = '';

    if ( sell_media_has_multiple_attachments( $id ) ) {
        $html .= sell_media_gallery( $id );
    } else {
        $html .= '<div class="sell-media-item-container sell-media-align' . $align . ' ">';
            $html .= sell_media_item_icon( $id, $size, false );
            $html .= '<p class="collection">' . sell_media_get_taxonomy_terms( 'collection' ) . '</p>';
            $html .= '<div class="sell-media-quick-view sell-media-button" data-product-id="' . esc_attr( $id ) . '" data-attachment-id="' . esc_attr( $attachment ) . '">' . $text . '</div>';
        $html .= '</div>';
    }

    return $html;
}