I think you don't fully understand what you're being told.
Just to clear things up:
your girlfriend or the player who purchased the rank will not be permanently banned. If the player who bought the rank charges the payment back, both the player who purchased the rank and the player who received the rank will be permanently banned. I am sure that this rule applies to all, if not almost all, Minecraft servers.
You cannot refund a rank or any package from Buycraft for your money back or for another package; the only thing you will receive from charging back is a permanent ban. However, very rarely, a rank can be switched from one count to another but I believe that only applies if both accounts are owned by the same person.
None of this is unfair; you cannot purchase something and charge back for it later just because you don't want it anymore. Yes, I understand that Steam allows you to request refunds, but MineVast is not Steam. All purchases are final unless under very special circumstances.
If this truly is that big of an issue, you can contact MV by emailing
[email protected]. Alternatively, you can add live:minevast on Skype for assistance.