shipping vs. payment relations??

Hi all,

is there any method how to define dependency between shiping method and payment method?
Example: I am using three methods of payment - COD, bank transfer in advance (proforma invoice) and cash when customer picks goods in my shop. Of course, cash is not applicable for delivery by post, COD is not applicable for personal pick up..

I guess that easiest way how to solve problem in my conditions is to use zone shipping module with zones for delivery by post with COD, delivery by post with payment in advance. In both cases there are post fees depended on weight plus surcharge for COD (which depends on total amount of money, but COD fees are not mayor problem, most of orders are less than limit for higher COD surcharge, for orders over this limit isn't important to pay somethig little more for COD). So I'll use three "geo-zones", one will be defined for postage without COD, one for postage with COD surcharge and one with zero cost for personal pick ups. But in this case the next step in checkout process is redundant, because payment method is sufficiently defined by shipping zone choice. It will needs some hack in checkout procedure code, that after shipping method choice will follow checkout confirmation instead of payment selection.
Does anyone solve something simillar? Any ideas?

Thanks,

Jiri