eCart can be used for international sales. Your merchant account may limit the types of credit cards that can be accepted. An international customer may try to use a credit card type that is not supported by your merchant account.
yes, international customers should use the State field for the Provence. All counties have postal codes, they may call it something else, but like the state, the zip code field is the closest for them to use.
to test an international order, you will need to modify the local checkout server behavior to use testmode. The local checkout server behavior will be on the confirm page.
you will also need to set your account to use test mode. You need to use tetmode so that the test credit car will not be checked against the billing address.
to troubleshoot why the transaction may be failing, see the "Debugging eCart transaction failed issues" tutorial on the eCart support page: