We have federal requirements that require that we ship certain items to a certified address. This means if you choose item1 and it has this limitation and it is in the cart we need to have a a special selection happen. So you would add to the cart.. put info in.. CC info etc.. then if an item of this type is in the cart we want a special section/page where they choose a location to have that special item ship to (this is outside of their standard shipping address) .. the special location list would be from a list or even better a web service based on their shipping zip code.. (the we service is already in place externally).
So the question is after you put in a valid credit card and move to next page can we have this special screen? I don't know here / how to add this..
I assume we can do this with PHP .. to call the web service.. the question is who can be go about getting the shipping zip of the customer and add the page in the checkout process?
For your requirements, you need to develop a Magento extension in which can do something like this:
Select the Magento event which gets executed after the payment is made.