Franco, I looked back at my rules and here is what we captured as far as using a web service or application for the back end.
When data is being passed (not owned or maintained by the system being counted) to a web service or another layer, we consider these systems as part of the same system so in this case we consider an ILF, and EI/EO/EQ.
When data is stored or referenced in the system being counted and in turn the data is passed to a web service or another application, we consider these systems as separate so in this case we consider an ILF and EI/EO/EQ for the main system and also EI/EO for the communication with the other system (or user in this case). Inoltre, an EIF is considered for the permanent data returned to the main system.
So for the above example, it would fall into my second paragraph since the address entered is stored in the main system and passed on to USPS for validation (and once confirmed is replaced with the address from USPS or not depending on the users response). So in this case the EI/EO would be considered along with the EIF.