It's just a work in progress at the moment, but I'd appreciate some help, again.

I'm writing a program for a delivery service where there are following conditions.

For orders worth less than 25 the delivery charge is 3.00.
Orders worth at least 25 but less than 100 are delivered free.
Orders worth 100 or more are delivered free and in addition the
customer receives a free gift. I prompt the user to enter the order value, and tell him/her which band he comes under.

This is what I have so far, and it doesn't seem to be it, evidently.

Code:
<html> 
<body> 
<script type='text/javascript'> 
// A delivery charge program 
var ordervalue; // how much the order is worth ordervalue = window.prompt ('How much is the order worth in Pounds Sterling?','');  
str='The value of your order is ' + ordervalue; 
if (ordervalue <= 25) 
str='Your delivery will cost 3 as your order value is ' + ordervalue'; 
if (ordervalue > 25 &&  ordervalue < 100)
{  str='Your delivery is free as your order value is ' + ordervalue';  
}else 
str='You will receive a free gift and delivery is free as your order value is ' + ordervalue; 
alert(str); // to output a result to see it is works! 
</script> 
</body> </html>
I'd love some help, please