Acmi48
Easy - Ignore the bill!
I worked for a flight planning company who simply just chucked it in the bin if we ever got such a bill and never took any flak for doing so!
I suspect you're getting these bills for ATC filing to H????ZPZX or ZTZX or any other "H" coded aftn address ?
the other way around this is to give your local agent at the airport that's ICAO starts with "H" the ATC plan and let him file it locally or if going in or out of Europe when filing to IFPU also send it as an IFPS (Supplement addresses)and add the "H"AFTN addresses you wish to file to .
Then E/CTRL will send it to them(My understanding is that E/Ctrl do not get charged for these addresses nor does any other AFTN address that ends ZPZX/ZTZX/YAYX etc etc)