Are you sure there are no such standards? I mean - in Czech Republic we have de facto standard called ISDOC XML(which should be based on other international standard UBL (universal bussines language, or smtg. like that)). All major accounting/finance software support it and yet, nobody uses it (at least those I have to deal with). I couldn't even convince my ISP to send me these files. Maybe you should check current options and then require them from your vendors.