Wordpress plugin / javascript for mileage/price calculator

For a courier website, I'm looking for a code which allows potential customers to get a quick quote. They would insert the postcode from where they want the item collected and the postcode to where they want the item delivered. The two postcodes would then generate an estimated price based on the driving distance between the postcodes.

We have several vehicles for use, each with their own rate, so depending on which vehicle the customer has opted for, the quote would be based on that vehicle.

Some examples:

http://www.onceoffdeliveries.ie/
http://www.theaa.com/driving/mileage-calculator.jsp

There also needs to be a minimum charge for mileages that are under a set number, ie 30 miles.

I'm using WordPress to develop the website.