Weight-Based Flat Rate Shipping Cost

$wpide_weight_tiers = array( // Max weight => cost.
    2  => 4.95,
    5  => 6.50,
    10 => 7.95,
);
$wpide_weight_extra = 0.50; // Cost per weight unit above the last tier.

add_filter( 'woocommerce_package_rates', function ( $rates, $package ) use ( $wpide_weight_tiers, $wpide_weight_extra ) {
    $weight = 0.0;

    foreach ( (array) $package['contents'] as $item ) {
        $weight += (float) $item['data']->get_weight() * (int) $item['quantity'];
    }

    ksort( $wpide_weight_tiers );
    $cost = null;

    foreach ( $wpide_weight_tiers as $max => $price ) {
        if ( $weight <= $max ) {
            $cost = $price;
            break;
        }
    }

    if ( null === $cost ) {
        $last = array_key_last( $wpide_weight_tiers );
        $cost = $wpide_weight_tiers[ $last ] + ceil( $weight - $last ) * $wpide_weight_extra;
    }

    foreach ( $rates as $rate ) {
        if ( 'flat_rate' !== $rate->get_method_id() ) {
            continue;
        }

        $old = (float) $rate->get_cost();
        $rate->set_cost( $cost );

        if ( $old > 0 && $rate->get_taxes() ) {
            $taxes = array();

            foreach ( $rate->get_taxes() as $key => $tax ) {
                $taxes[ $key ] = $tax * $cost / $old;
            }

            $rate->set_taxes( $taxes );
        }
    }

    return $rates;
}, 20, 2 );