Example of how you can add a settings fields, and retrieve the value in the shortcode template.

external-product-embed-extensibility.php

<?php

/**
 * Add Product Settings Section to the Admin
 */
add_action( 'wcepe_after_transients_settings', 'wcepe_custom_settings_section' );

function wcepe_custom_settings_section() {

	// Transient Section
	add_settings_section(
		'wcepe_products_settings_section', 
		__( 'Product Settings', 'woocommerce-external-product-embed' ), 
		'wcepe_product_settings_instructions','wcepe_settings_group'
	);

	// Consumer Secret
	add_settings_field( 
		'wcepe_extra_button',
		__( 'Extra Button Link', 'woocommerce-external-product-embed' ),
		'wcepe_product_settings_text_field', 'wcepe_settings_group', 'wcepe_products_settings_section'
	);

}

function wcepe_product_settings_instructions() { 

	echo "Configure the Product Settings";

}

function wcepe_product_settings_text_field() {

	$options = get_option( 'wcepe_settings' ); ?>

	<input type='text' class="regular-text wcepe_extra_button" name='wcepe_settings[wcepe_extra_button]' value='<?php echo $options['wcepe_extra_button']; ?>'>
	
	<?php

}

// End Admin Settings


/**
 * Add Extra Button front-end product array
 */
add_filter( 'wcepe_filter_product', 'wcepe_add_extra_button' );

function wcepe_add_extra_button( $product ) {

	$product['extra_button'] = 'testing';

	return $product;

}

// End Front-End Settings



/**
 * Add the following line of code wherever you would like to in the template to show this contents of this newly created field:
 *
 * <a href=<?php echo $product['extra_button'] ?> class="" target="_blank"><?php echo "Visit Store" ?></a>
 */

Results

Settings: http://cld.wthms.co/1achx/4CLyaJMO
Front-end: http://cld.wthms.co/1eAvT/2YDynWSS