Suggestions for this plugin

1. You could add a redirect rule to htaccess to replace apply_coupon with something else.

2/3. This is a snippet I use in my child theme to override certain texts:

$qrvm_2015_catch_translations = array();
add_action( 'after_setup_theme', 'qrvm_2015_setup' );
function qrvm_2015_setup() {
    load_child_theme_textdomain( 'qrvm_2015', get_stylesheet_directory() . '/languages' );

	global $qrvm_2015_catch_translations;
	$qrvm_2015_catch_translations = array (
		'woocommerce-jos-autocoupon' => array(
			'Coupon code already applied!' => __( 'YOUR TEXT', 'qrvm_2015'),
			'Coupon code applied successfully.' => __( 'YOUR TEXT', 'qrvm_2015'),
		),
	);

}

//Override translations
add_filter('gettext', 'qrvm_2015_filter_gettext', 10, 3);
function qrvm_2015_filter_gettext($translation, $text, $domain) {
	global $qrvm_2015_catch_translations;
	$my_domain = 'qrvm_2015';

	if (isset($qrvm_2015_catch_translations[$domain])) {
		if ( isset( $qrvm_2015_catch_translations[$domain][$text] ) ) {
			return $qrvm_2015_catch_translations[$domain][$text];
		}
	}
	return $translation;
}

Replace the entries in the qrvm_2015_catch_translations-array
Then use poedit or some other tool to create en_EN.po files in the languages folder of your theme if you want multilingual support.

Thanks for your quick reply.

1. Actually, I want to allow use to apply code with below format. Any suggestion? Thanks.
https://www.abc.com/product/abc?pr=coupon_code

2/3. Added the below code to functions.php of my child theme, but not change for the notice message, tried clear cache. Please advise. Thanks.

/* Override URL to Coupon Notice */
$qrvm_2015_catch_translations = array();
add_action( 'after_setup_theme', 'qrvm_2015_setup' );
function qrvm_2015_setup() {
    load_child_theme_textdomain( 'qrvm_2015', get_stylesheet_directory() . '/languages' );
	global $qrvm_2015_catch_translations;
	$qrvm_2015_catch_translations = array (
		'woocommerce-jos-autocoupon' => array(
			'Coupon code already applied!' => __( 'Coupon code already applied. Discount reflected at cart checkout!', 'qrvm_2015'),
			'Coupon code applied successfully.' => __( 'Congratulations, Coupon code applied successfully. Discount reflected at checkout!', 'qrvm_2015'),
		),
	);
}
//Override translations
add_filter('gettext', 'qrvm_2015_filter_gettext', 10, 3);
function qrvm_2015_filter_gettext($translation, $text, $domain) {
	global $qrvm_2015_catch_translations;
	$my_domain = 'qrvm_2015';

	if (isset($qrvm_2015_catch_translations[$domain])) {
		if ( isset( $qrvm_2015_catch_translations[$domain][$text] ) ) {
			return $qrvm_2015_catch_translations[$domain][$text];
		}
	}
	return $translation;
}

1 you can use htaccess to redirect (google it). Otherwise look in wjecf-autocoupon.php and find apply_coupon.

2/3 In the snippet replace woocommerce-jos-autocoupon by woocommerce.

Hi Soft79,

It works, and thanks for your great help!

1.
Could you please also give me hints on which file/function I should look into to make the below sentence shown in notice form (green), instead of error form (red)?
https://screencast.com/t/3uzIZPolbhMh

2.
For notification, I saw a room for improvement that we could customize the notice for each coupon applied via URL to specify how it work: % Off, Free Shipping or $ OFF. I suggest to add text fields in coupon edit page for them.
You could consider to add it to PRO version and I am happy to purchase the PRO version with it. Thanks.
Example:
“Congratulations, you’ll be receiving 5% off your purchase. Discount reflected at checkout!”
https://www.teezily.com/test1213213123?pr=fr5e

1. The notice is created in class-wc-cart.php of WooCommerce (look for `$the_coupon->add_coupon_message( WC_Coupon::E_WC_COUPON_ALREADY_APPLIED );
`).

2. Sounds like a nice idea. I might implement in a future version, but has no priority. For now you can use the filter woocommerce_cart_totals_coupon_label or woocommerce_cart_totals_coupon_html.

Thanks for your info.

1. Could you give me more hint on how to override it? Need to do in direct edit class-wc-cart.php?

Thanks.