Add discount to woocomerce cart page INCL. tax - php

Question

I am trying to modify the code based on this question:
// Display the checkout field in cart page totals section
add_action( 'woocommerce_cart_totals_before_order_total', 'display_priority_fee_checkbox_field', 20 );
function display_priority_fee_checkbox_field(){
echo '<tr class="installment-section">
<th>'.__("Priority Dispatch").'</th><td>';
woocommerce_form_field( 'priority_fee', array(
'type' => 'checkbox',
'class' => array('form-row-wide'),
'label' => __(' $20.00'),
), WC()->session->get('priority_fee') ? '1' : '' );
echo '<div class="tooltip">?
<span class="tooltiptext">'.__("By selecting this option... ").'</span>
</div></td>';
}
// Remove "(optional)" text from the field
add_filter( 'woocommerce_form_field' , 'remove_optional_txt_from_priority_fee_checkbox', 10, 4 );
function remove_optional_txt_from_priority_fee_checkbox( $field, $key, $args, $value ) {
// Only on checkout page for Order notes field
if( 'priority_fee' === $key ) {
$optional = ' <span class="optional">(' . esc_html__( 'optional', 'woocommerce' ) . ')</span>';
$field = str_replace( $optional, '', $field );
}
return $field;
}
// jQuery :: Ajax script
add_action( 'wp_footer', 'priority_fee_js_script' );
function priority_fee_js_script() {
// On Order received page, remove the wc session variable if it exist
if ( is_wc_endpoint_url('order-received')
&& WC()->session->__isset('priority_fee') ) :
WC()->session->__unset('priority_fee');
// On Cart page: jQuert script
elseif ( is_cart() ) :
?>
<script type="text/javascript">
jQuery( function($){
if (typeof woocommerce_params === 'undefined')
return false;
var c = 'input[name=priority_fee]';
$(document.body).on( 'click change', c, function(){
console.log('click');
var fee = $(c).is(':checked') ? '1' : '';
$.ajax({
type: 'POST',
url: woocommerce_params.ajax_url,
data: {
'action': 'priority_fee',
'priority_fee': fee,
},
success: function (response) {
setTimeout(function(){
$(document.body).trigger('added_to_cart');
}, 500);
},
});
});
});
</script>
<?php
endif;
}
// Get Ajax request and saving to WC session
add_action( 'wp_ajax_priority_fee', 'priority_fee_ajax_receiver' );
add_action( 'wp_ajax_nopriv_priority_fee', 'priority_fee_ajax_receiver' );
function priority_fee_ajax_receiver() {
if ( isset($_POST['priority_fee']) ) {
$priority_fee = $_POST['priority_fee'] ? true : false;
// Set to a WC Session variable
WC()->session->set('priority_fee', $priority_fee );
echo $priority_fee ? '1' : '0';
die();
}
}
// Add a custom calculated fee conditionally
add_action( 'woocommerce_cart_calculate_fees', 'set_priority_fee' );
function set_priority_fee( $cart ){
if ( is_admin() && ! defined('DOING_AJAX') )
return;
if ( WC()->session->get('priority_fee') ) {
$item_count = $cart->get_cart_contents_count();
$fee_label = 'Discount';
$fee_amount = -2;
$cart->add_fee( $fee_label, $fee_amount );
}
}
But the discount is not showing up. Also, I nneed the discount to be applied to the SUBTOTALS so that tax is recalculated as well (which the code does not do, it applies the discount after taxes).
How can I set a negative fee(discount) when checkbox is checked and also have the discount applied before tax calculation?
EDIT: Okay, I got it to work with the taxes by changing the last part:
// Set the discount
add_action( 'woocommerce_cart_calculate_fees', 'checkout_set_discount', 20, 1 );
function checkout_set_discount( $cart ) {
if ( ( is_admin() && ! defined('DOING_AJAX') ) )
return;
$subtotal = WC()->cart->get_subtotal();
$percentage = 1;
$discount = $subtotal * $percentage / 100;
if( WC()->session->get('priority_fee') ) {
$cart->add_fee( sprintf( __( 'Discount', 'woocommerce') ), -$discount );
}
}
but this only works with a percentage based discount. How can I instead use a fixed discount?