I second these suggestions. Another real-world-example from me then:
When a customer chooses COD (Cash on Delivery is a shipping-method at the Royal Norwegian Postalservice) which has a surcharge to it, it costs more than sending an ordinary package, becayse the Postalservice charges you for accepting payment on your behalf. This means that we have to have a spesific payment-method for this (lets call it COD as well), which has this surcharge attached.
Now these dependancies go both way, whatever comes first doesn’t really matter - the importance in this example is to say thay if you choose COD, you only have -1- payment-method (or shipping-method) available.
Hope I didn’t ramble too much and that my point got through