Product page custom checkbox enable a percentage discount on WooCommerce cart subtotal

The goal is simple, add a checkbox to the WooCommerce Product Page. If the user checks the checkbox, a 5% discount is applied on cart subtotal.

I have checked and doubled checked the code like ten times, I just cannot figure out why it gives me the "Fatal Error".

This is the code:

add_action( 'woocommerce_after_add_to_cart_button', 'ls_automatic_discount_checbox', 10 );
function ls_automatic_discount_checbox() {
    echo '<p><label><input type="checkbox" name="apply_automatic_discount" value="1"/> Apply 5% discount to your subtotal</label></p>';
}

add_action( 'woocommerce_cart_calculate_fees', 'ls_apply_discount_on_checkbox_check', 10, 7 );
function ls_apply_discount_on_checkbox_check( $cart, $cart_item_key, $product_id, $quantity, $variation_id, $variation, $cart_item_data ) {

        // makes sure that the checkbox is ticked
        if ( isset( $_POST[ 'apply_automatic_discount' ] ) && $_POST[ 'apply_automatic_discount' ] == 1 ) {        

        // if it is, check what the percentaged discount on the subtotal is
        $percent = 5;

        // if all checks out - give the discount
        if ( isset ( $percent ) && $percent > 0 ) {

        // customer gets a discount of 5% of the SUBTOTAL
        $discount = $cart->cart_contents_total * $percent / 100;
        $cart->add_fee( __('5% OFF', 'woocommerce' ) . " (" . $percent . "%)", -$discount);
    
        }
    }
}

There are some missing things and mistakes in your code:

• 1st, you need to save the checkbox value as custom cart item data.
• the hook woocommerce_cart_calculate_feeshas only one argument: $cart (the WC_Cart object). That was the main error issue.
• You could replace woocommerce_after_add_to_cart_button hook (that displays the checkbox after add to cart button) with woocommerce_before_add_to_cart_button, to display the checkbox before, which is more logical.

The code:

add_action( 'woocommerce_after_add_to_cart_button', 'ls_automatic_discount_checbox', 10 );
function ls_automatic_discount_checbox() {
    printf( '<p class="auto-discount"><label><input type="checkbox" name="auto_discount" value="1"/> %s</label></p>', 
        __('Apply 5% discount to your subtotal', 'woocommerce') );
}

// Add custom cart item data
add_filter('woocommerce_add_cart_item_data', 'add_custom_field_value_to_cart_item_data', 10, 2);
function add_custom_field_value_to_cart_item_data($cart_item_data, $product_id) {
    $cart_item_data['auto_discount'] = isset($_POST['auto_discount']) ? '1' : '0';
    return $cart_item_data;
}

Now you can add a discount globally or by product (where the checkbox has been checked):

A/. Globally (on the cart subtotal):

add_action( 'woocommerce_cart_calculate_fees', 'auto_applied_product_discount', 10 );
function auto_applied_product_discount( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    $auto_discount = false; // Initializing

    // Loop through cart items
    foreach ( $cart->get_cart() as $cart_item ) {
        if ( isset($cart_item['auto_discount']) && $cart_item['auto_discount'] ) {
            $auto_discount = true;
            break; // Stop the loop
        }
    }
    
    if ( $auto_discount ) {
        $percent  = 5;
        $total    = $cart->cart_contents_total;
        $cart->add_fee( __('5% OFF', 'woocommerce' ) . " (" . $percent . "%)", -($total * $percent / 100));
    }
}

B/. By product subtotal (where the checkbox has been checked):

add_action( 'woocommerce_cart_calculate_fees', 'auto_applied_product_discount', 10 );
function auto_applied_product_discount( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    $total = 0; // Initializing

    // Loop through cart items
    foreach ( $cart->get_cart() as $cart_item ) {
        if ( isset($cart_item['auto_discount']) && $cart_item['auto_discount'] ) {
            $total += $cart_item['line_total'];
        }
    }
    
    if ( $total > 0 ) {
        $percent  = 5;
        $cart->add_fee( __('5% OFF', 'woocommerce' ) . " (" . $percent . "%)", -($total * $percent / 100));
    }
}

Code goes in functions.php file of your child theme (or in a plugin). Tested and works.