Hi Jens,
Ok firstly your understanding of start conditions is not correct. A start conditions acts as a qualifier to an event, so that the SAME event can be used but with different start conditions to control which workflow gets started and/or which web service gets sent to PI.
So you could use your "customer changed" event to do both provided the customer object holds the old and new values of the change to test in the start condition.
Perhaps what you are thinking of is transaction SWEC where workflow events can be triggered based on change documents - so that you have the before and after image available to determine which workflow event gets raised and with what details. To my mind this would be a better approach for your requirement. If it was me, the development cost of defining another event is very minimal. So I would be inclined to create additional events for "customer new postcode added" (with the change document condition postcode field was blank before and is now filled) and "customer postcode removed" (with the change document condition postcode field was filled and is now blank). You can do this provided postcode is tracked in the change documents.
This way when your customer is changed you always get your standard "customer changed" event, but if and ONLY if the postcode was changed you also get one of your customer postcode events as well. You can then separate out the postcode dependent functionality to the new events.
You can of course write code, but why code when you can configure? Not only do you save on development time, but you leave a more understandable, easier to support, easier to adapt system behind you.
Regardless I'm thinking that whatever answer you come up with should be worth blogging about... :-)
Regards,
Jocelyn