"Squash" all delivery options into a single one

functions.php

<?php
// Paste everything below this line to your child-theme's functions.php file.

// "Squashes" all delivery options into a single one.
// After pasting this snippet, reset the WooCommerce shipping cache, e.g. add an item to the cart.
add_filter('woocommerce_package_rates', function ($rates, $package) {
    if (count($rates) > 1) {
        $firstRate = null;
        $firstRateId = null;
        foreach ($rates as $id => $rate) {
            if (!isset($firstRate)) {
                $firstRate = $rate;
                $firstRateId = $id;
            } else {
                $firstRate->cost += $rate->cost;
            }
        }
        $rates = [$firstRateId => $firstRate];
    }
    return $rates;
}, 10, 2);