So we are talking about financial transactions that need to be securely tranfered. This FTPS is what the company that we have chosen has to offer, so we dont have a choice and no they dont provide a vpn solution.
File has to be created and encrypted , sent across to their FTPs server.
There seems to be a confusion regarding the type of transfer
FTP = File transfer protocol
This is unencrypted and it not possible to properly secure the transfer with this protocol.
FTP over SSL This is just normal FTP encrypted with SSL/TLS with all the drawbacks that comes with that solution.
This is a firewall unfriendly solution.
SSH File Transfer protocol
This is "secure" from the start and firewall friendly.
I have done several of these solutions before and as a general point when you raise the concerns you have they tend to listen and make a solution available.
So if they have FTP server as you state in your first post ,then there is no way to do this in a secure manor.
The best you can do is encrypt the file itself but that is at best a questionable solution.
If the Server is FTPS as you stat in your second post then the protocol in itself is encrypted and you are "secure" but you will have to open much more ports than you would like to open in the firewalls.
One thing you could do to circumvent the problem is that you are getting a SFTP server on your end and let them come and get the file there.