Trouble with shipping options

I have following problem.

My customers can choose two different payment methods: credit card or cash on delivery.

I have set up my store so you will first choose payment method (CC or COD).

And after that the shipping method. The problem is that COD is more expensive for me so shipping cost with COD are higher. Due to this customers often choose COD as a payment method and normal (cheaper) shipping method. Is there any way that I could set up system so that when you choose COD you can no longer choose normal shipping, but only COD shipping? I have written info about this at the order page, but of course people won't notice this.

Any ideas?

Not familiar with COD but does it have a setting for tare? This is where you can add what's needed to make up the cost.

If "properly" reworked, the payment module information should be in the session data, global $payment variable, or $order variable, therefore in the applicable "normal" shipping method, the enabled variable ought to be able to be set to false if COD payment is the selected payment. A similar "logic" could have been applied in the standard processing sequence, but instead against the chosen shipping method.

It would be of better/more direct assistance if there were more specifics provided.

^What info do you need?