post drop down box value

[rob_cart]

I have three drop down boxes which display depending on the state of 3 radio buttons. what i am wanting is to post the value of the drop down box to my next page but only the value of the drop down box which is displayed, not the other 2 that are currently hidden.
I think i would need to transfer the value of the visable drop down box into a hidden text field but i dont really no how !!

Thanks a lot