Hide Shipping Methods Based on Cart Total

add_filter( 'woocommerce_package_rates', function( $rates, $package ) {

    $threshold = 3000;

    if ( WC()->cart->subtotal >= $threshold ) {
        foreach ( $rates as $rate_id => $rate ) {
            if ( 'flat_rate' === $rate->method_id ) {
                unset( $rates[ $rate_id ] );
            }
        }
    }
    return $rates;

}, 10, 2 );