Draft for Direct Stripe input coupons field

ds-input-coupon.php

<?php
/*
 * Plugin Name: Direct Stripe input coupons
 * Author: Nicolas Figueira
 * Description: Allow users to use promotional coupons with Direct Stripe
 * Version:     0.0.1
 * Text Domain: ds-coupons
 *
 * This is not production ready
 * It will add an input field for coupon with CSS ID button_with_coupon
 * Apply any coupon ID inserted as the input value as a coupon disount for the subscription
 * I recommend leaving the Error message option empty in the Global settings, this will show a coupon does not exist error when this is the case.
 */

/**
 * Set script for Direct Stripe coupon
 * We Insert the JS code that will read the input field value in the footer using wp_footer action hook
 *
 */
add_action( 'wp_footer', function(){
	?><script type="text/javascript">
	jQuery(function($) {

		//Get value of coupon input field
		$("#button_with_coupon").on("click", function() {
			var coupon = $(".input-coupon").val();

			$.post(
				ds_coupons_vars.ajaxurl,
		      {
			'action': 'ds_set_coupon',
			'coupon': coupon
		      }
		    );
		  });

	});
    </script>
	<?php
}, 10 );


/**
* Add an input field before the button
* We use 'direct_stripe_before_button' action hook to insert the html input field(s) required
*/
add_action( 'direct_stripe_before_button', function( $button_id ) {
    if( $button_id === 'button_with_coupon') {
      ?>
        <div class="coupon-input-field">
            <label><?php _e('Do you have a coupon ?', 'ds-coupons'); ?></label>
            <input type="text" class="input-coupon"></input>
            <span class="ds-coupons-answer" style="display:none;"></span>
        </div>
        <?php
    }
});


/**
* SET Coupon value as a transient for process
* The ds_set_coupon PHP function is triggered by WordPress ajax hooks 
* in our case we called the wp ajax url with the action 'ds_set_coupon' in our wp_footer hook that printed the JS 
* and ajax hooks are calling php function with the same name ( could have been a different name )
*/
add_action( 'wp_ajax_ds_set_coupon', 'ds_set_coupon' );
add_action( 'wp_ajax_nopriv_ds_set_coupon', 'ds_set_coupon' );
function ds_set_coupon() {
    $coupon = isset($_POST['coupon']) ? $_POST['coupon'] : '';
    if( isset( $coupon ) ) {
        set_transient("ds_coupon_value", $coupon );
    }
    wp_die();
}

/**
* Set the coupon value in process Stripe API data
*
* We use 'direct_stripe_subscription_data' filter hook to add the coupon value stored as a transient to the data sent to Stripe API
*/
add_filter( 'direct_stripe_subscription_data', function( $subscriptiondata, $user, $token, $button_id, $amount, $coupon ) {

    if( $button_id === 'button_with_coupon' ) {


        $coupon_input = get_transient('ds_coupon_value');
        if( isset( $coupon_input ) && $coupon_input !== false && !empty( $coupon_input ) ) {
            $subscriptiondata['coupon'] = $coupon_input;
        }
    }
    return $subscriptiondata;
}, 10, 6 );