napominanie,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
var i=0;
var j = i;
</script>
</head>
<body>
<form name="shop" action="http://">
<input name="" type="checkbox" onclick="j = this.checked ? i + 300 : i">
<input type="button" value="заказать" onclick="document.shop.area.value=j">
<textarea name="area" id="" cols="5" rows="2"></textarea>
</form>
</body>
</html>