Add-ons integration not working properly when changing WC Quantity field to select

[Valentina R]

This user changed the default WC quantity field to select and the add-ons with flat based price work as quantity based instead.

Tested on a simple product and it works fine, but for appointable products, it doesn’t work ok:

This is the code used in “mytheme-child/woocommerce/global/quantity-input.php”:

<?php
/**
 * Product quantity inputs
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/global/quantity-input.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see     https://docs.woocommerce.com/document/template-structure/
 * @package WooCommerce\Templates
 * @version 4.0.0
 */

defined( 'ABSPATH' ) || exit;

if ( $max_value && $min_value === $max_value ) {
    ?>
    <div class="quantity hidden">
        <input type="hidden" id="<?php echo esc_attr( $input_id ); ?>" class="qty" name="<?php echo esc_attr( $input_name ); ?>" value="<?php echo esc_attr( $min_value ); ?>" />
    </div>
    <?php
} else {
    /* translators: %s: Quantity. */
    $label = ! empty( $args['product_name'] ) ? sprintf( esc_html__( '%s quantity', 'woocommerce' ), wp_strip_all_tags( $args['product_name'] ) ) : esc_html__( 'Quantity', 'woocommerce' );
    ?>
    <div class="quantity">
        <?php do_action( 'woocommerce_before_quantity_input_field' ); ?>
        <label class="screen-reader-text" for="<?php echo esc_attr( $input_id ); ?>"><?php echo esc_attr( $label ); ?></label>
        <?php
        if ( is_cart() ) :
        ?>
        <input
            type="hidden"
            id="<?php echo esc_attr( $input_id ); ?>"
            class="<?php echo esc_attr( join( ' ', (array) $classes ) ); ?>"
            name="<?php echo esc_attr( $input_name ); ?>"
            value="<?php echo esc_attr( $input_value ); ?>"
            title="<?php echo esc_attr_x( 'Qty', 'Product quantity input tooltip', 'woocommerce' );
        ?>" />
        <?php
        endif;

        $options = ''; // Initializing

        for ( $i = $min_value; $i <= $max_value; $i += $step ) :
            $selected = ( '' !== $input_value && $input_value >= 1 && $i == $input_value ) ? 'selected' : '';
            $options .= '<option value="' . $i . '"' . $selected . '>' . $i . '</option>';
        endfor;
        // Change input name on select field
        $attr_name = is_cart() ? 'data-name' : 'name';
        ?>
        <select <?php echo $attr_name; ?>="<?php echo $input_name; ?>"><?php echo $options; ?></select>
        <?php do_action( 'woocommerce_after_quantity_input_field' ); ?>
    </div>
    <?php
}

Reported here.