So... FileMaker generates the csv...
And your issue is when uploading to some random third party website?
I fear you have lost your way, you may consider posting to Royal Mail's forum, if they have one.
I have not lost my way it seems theres something in the way that the CSV is generated that some windows based PC's don't see the files data in less you open with a PC based version of excel and save again, I have seen there are differet ways of importing and exporting the data, i just need to know if anyone else has ever had this issue
What file extension is produced when you export from Filemaker?
the extension is .csv