Google maps api automatic route generator

Various customers over the USA place orders with us. When we receive an order It has a FROM location and a TO location. At the moment, I have to take these addresses and manually use google maps directions to plan out the most efficient route based on time. Typically there are 15-20 FROM and TO locations per trip that I do. I require preferably a webscript (if not then an easy to use desktop app) so that I can input the TO and FROM address's and this will automatically calculate the most efficient route without me having to work this out manually. Also when orders come in during the week if a trip is already ongoing then sometimes it is necessary to ammend the current trip to addon new TO / FROM addresses. If you have any queries then please don't hesitate to ask. The order site at present is coded in PHP and the address's of the TO / FROM locations are all stored backend in an mysql db. I am currently working on a new site using AJAX so either delivery method is acceptable.