DarkMilkMan,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<form name="frm" action="http://">
<select name="weightprim" >
<option value="0">-- Выберите вес товара --</option>
<option value="100">до 1 кг</option>
<option value="200">1 - 3 кг</option>
<option value="300">3 - 5 кг</option>
<option value="400">5 - 10 кг</option>
<option value="500">10 - 25 кг</option>
<option value="600">25 - 100 кг</option>
</select>
<label for="dostdobel">Стоимость доставки <u>до склада в Беларуси</u> в бел.руб.:</label><input type="number" id="dostdobel" name="dostdobel" maxlength="300" value="0" readonly><br>
</form>
<script>
(function(){
var form = document.forms.frm;
form.onchange = form.oninput = calcTotal;
function calcTotal() {
form.dostdobel.value = form.weightprim.value;
}
})();
</script>
</body>
</html>