The invoice must be assignable unequivocally to a corresponding oder submitting author at Freie Universität Berlin. The FU affiliation must be clearly recognizable in the publication and on the invoice. The authors are responsible for the correct indication.

Yes. However, the author's Freie Universität address has to be given as official address – either exclusively or in combination with another address.

The relevant question is only whether the publishing journal is a "genuine" open-access journal. As a rule, such a journal will be listed on http://www.doaj.org/. It is of no importance through which additional websites the article is channeled.

Even if journals have not (yet) been registered with the DOAJ, publication fees can be covered through the publication fund. In all cases, journals must be genuine open-access journals, and according to the regulations, they must have strict quality-assurance procedures in place.