<html>
<head>
</head>
<body>
<meta charset="windows-1251" />
<select name="shk_delivery" style="width:99%;">
<option id="delivery1" value="delivery1" selected="selected" >delivery1</option>
<option id="delivery2" value="delivery2" >delivery2</option>
<option id="delivery3" value="delivery3" >delivery3</option>
</select>
<select id="test">
<option>350</option>
<option>1050</option>
<option>1700</option>
<select>
<div id="total_price" style="text-align:right;">Общая сумма: <b>350</b> руб.</div>
<script>
test.onchange=function(){
document.querySelector('#total_price b').innerHTML=this.value
}
onload=document.querySelector("select").onclick=function(){
var n=+document.querySelector('#total_price b').innerHTML
if(n<1000) delivery2.disabled=true
if(n>1000) delivery2.disabled=false
if(n<1500) delivery3.disabled=true
if(n>1500) delivery3.disabled=false
}
</script>
</body>
</html>