Iconic WDS - Dynamically update the fee based on the current day and same day

wds.php

<?php
add_action( 'woocommerce_cart_calculate_fees', 'iconic_wds_dynamically_add_custom_fee', 9 );
function iconic_wds_dynamically_add_custom_fee() {
	global $iconic_wds;
	if ( empty( $iconic_wds ) ) {
		return;
	}

	$chosen_method = $iconic_wds->get_chosen_shipping_method();

	// Todo change the fees.
	$same_day_fees_amount = 10;
	$saturday_fees_amount = 21;

	// If the chosen method is flat rate, set the fee.
	if ( strpos( $chosen_method, 'flat_rate' ) !== false ) {
		WC()->session->set( 'jckwds_same_day_fee', $same_day_fees_amount );
	} else {
		WC()->session->__unset( 'jckwds_same_day_fee' );
	}

	if ( ! isset( $_POST['post_data'] ) ) {
		return;
	}

	$checkout_fields = array();
	parse_str( $_POST['post_data'], $checkout_fields );

	if ( empty( $checkout_fields['jckwds-delivery-date-ymd'] ) ) {
		return;
	}

	$delivery_date_ymd = $checkout_fields['jckwds-delivery-date-ymd'];
	$delivery_date = DateTime::createFromFormat( 'Ymd', $delivery_date_ymd );

	// if $delivery_date is saturday, set the saturday fee.
	if ( $delivery_date->format( 'N' ) == 6 ) {
		WC()->session->set( 'jckwds_day_fee', $saturday_fees_amount );
	} else {
		WC()->session->__unset( 'jckwds_day_fee' );
	}
}