Delivery charge program

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