Including inline Custom Fields info with add_filter in functions.php

You are trying to echo within a variable. You also have encapsulated PHP tags in the string as well. You need to concatenate just the functions themselves like this:

    function default_content($content) {
        global $post;

        if ($post->post_type == 'my-custom-post-type') {
            $content .= '<p style="text-align: center;"><strong>Custom Field Text here: '. get_post_meta( $post->ID, "custom-field-1", true ).'</strong></p>
                         <p style="text-align: center;"><a href="http://myblog.com/?checkout=" . get_post_meta( $post->ID, "custom-field-2", true )."">Link 01</a></p>';
        }
        return $content;
    }
    add_filter('the_content', 'default_content', 0);

Also you are calling global $post, so use $post->ID instead of get_the_ID().

Cheers!