Apply Coupon code discount to the final order total - php

Question

We sell trails and the trails are "products" in WooCommerce. >>
Check this link
Try clicking on the "Let's Play" link and it will take you to the checkout. >> (Screenshot)
The price of each trail is set to zero (in WooCommerce admin) because the pricing is "per person".
We had to use the following code snippet to make the products "work", with the "$0" price.
add_filter( 'woocommerce_is_purchasable', 'wpa_109409_is_purchasable', 10, 2 );
function wpa_109409_is_purchasable( $purchasable, $product ){
if( $product->get_price() >= 0 )
$purchasable = true;
return $purchasable;
}
After making the products "work", with the "$0" price, we successfully added some required fields on the checkout page which are used for adding a "fee" for every "person", the visitor will select.
The order total will only update when the user will select the number of players field on the checkout page, notice the "Select No of Players" options.
(Screenshot)
The Issue:
Whenever we apply a "Coupon" (For example "enjoy10" - 10% discount), it only applies to the product price, which is currently set-to "$0".
(Check this Screen Record)
So, technically, the coupon code doesn't work and applied only "$0" discount because the coupon doesn't apply on the order total, it only applies to the product price, which is set to "$0", because we're charging on a per person basis.
I want the coupon code to apply to the final order total, not just the product price.

Answer 1

In default, the coupon will be applied to subtotal or on the product price. Subtotal means the total amount of product.
When you submit some additional fields in the checkout, the additional amount will be added as fee in WooCommerce. The fee is the same as the coupon. Using fee, we can increase or reduce the total amount. When we apply a coupon, WooCommerce applies a negative fee on the subtotal and that so the total amount is reduced.
The options possible are either change your coupon amount as per the latest fee or add a negative fee (additional discount) for the checkout field fee by calculating the amount of coupon.