Change a Product's True Quantity for Calculating Shipping

betrs-adjust-product-qty.php

/**
 * Adjust the calculated quantity of items in the cart based on a specific product
 * Code snippets should be added to the functions.php file of your child theme
 *
 * @return array
 */
function adjust_item_qty_for_shipping( $data, $items ) {
    // the Product ID of the product that needs to be adjusted
    $item_to_change = 13;

    // the multiplier for adjusting the quantity
    $multiplier = 5;

    // exit if no valid items are provided
    if( ! is_array( $items ) ) return $data;
    
    // check each product in the cart
    $qty_change = 0;
    foreach( $items as $key => $item ) {
        if( $item['product_id'] == $item_to_change || $item['variation_id'] ) {
            $qty_change = $item['quantity'] * $multiplier - $item['quantity'];
        }
    }

    // adjust the order's quantity
    $data['quantity'] = $data['quantity'] + $qty_change;

    return $data;
}
add_action( 'betrs_calculated_totals-per_order', 'adjust_item_qty_for_shipping', 10, 2 );