Thanks for flagging me, @George4Tacks!
@LL2The form will need to be paper-filed with the Ogden processing center. There are no other filing options available. Note that F.3520 often comes with other filing/reporting obligations as well as tax consequences your client may not have considered. If it's being filed only for the receipt of foreign gifts, it would be relatively straight forward even with FBAR and FATCA reporting.
Still an AllStar