Could someone please help me. I am desperate to get this working.

I am trying to create a calculator that can calculate the total cost of something using select menus. I have managed to do it with one item, no problem, but when it comes to calculating multiple items in one form, I get errors all over the place.

Here is an example of what I need;

Take a look at the select boxes at the bottom of the page and mess about with them and you will see what I mean. Feel free to mess with the source code aswell.

As I said it works fine with just one select box and one item, but not with multiple.

I would appreciate it greatly if someone could help me out here.