Oto Moja propozycja, oczywiście css i komunikaty przykładowe. 
dodaj.php
<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
$transport_z = (! empty($_POST['transport-z'])) ? $_POST['transport-z'] : null;
$transport_do = (! empty($_POST['transport-do'])) ? $_POST['transport-do'] : null;
$kurier = (isset($_POST['kurier'])) ? (int)$_POST['kurier'] : null;
if ($transport_z && $transport_do && is_numeric($kurier)) {
$kurierzy = array(
array('name'=>'DHL', 'email'=>'dhl@lgmail.com'), // index 0
array('name'=>'InPost', 'email'=>'inpost@lgmail.com') // index 1
);
$subject = "Transport z {$transport_z} do {$transport_do}"; // Tytuł e-maila
$body = "Transport z {$transport_z} do {$transport_do},
kurier {$kurierzy[$kurier]['name']}"; // Treść email-a
if (@mail($kurierzy[$kurier]['email'], $subject, $body)) {
$info = "<span>Twój wybór:</span>
<span><span> Transport z:</span> {$transport_z}</span>
<span><span> Transport do:</span> {$transport_do}</span>
<span><span> Kurier:</span> {$kurierzy[$kurier]['name']}</span>
<span><span>Wysłano email do:</span> {$kurierzy[$kurier]['email']}</span>";
} else {
$info = "<span class=\"info-error\">Wystąpił problem z wysłaniem email-a
do {$kurierzy[$kurier]['name']}!</span>";
}
} else {
$info = '<span class="info-error">Wystąpił problem z formularzem!</span>';
}
} else {
$info = '';
}
?>
<!DOCTYPE html>
<html lang="pl">
<head>
<meta charset="utf-8">
<style>
* {
box-sizing: border-box;
font: 300 1em monospace;
}
input, label, select {
display: block;
margin: 0.5em 0;
width: 16em;
}
.info {
margin-top: 2em;
}
.info span {
display: block;
}
.info span span {
display: inline-block;
font-weight: bold;
}
.info-error {
color: red;
}
</style>
</head>
<body>
<form class="transport" action="" method="post" id="transport">
<input type="text" name="transport-z" placeholder="Transport z" required>
<input type="text" name="transport-do" placeholder="Transport do" required>
<label for="kurier">Wybierz przewoźnika:</label>
<select name="kurier" id="kurier" required>
<option value="0" selected>DHL</option>
<option value="1">InPost</option>
</select>
<input type="submit" value="Wykonaj">
</form>
<div class="info">
<?php echo $info; ?>
</div>
</body>
</html>