In order to avoid Goko, I employ a third party to mail a hand to me and my opponent. We then mail back our moves and the third party mails back the result.
So let's say the arbiter mails me a Lab and four Coppers. I open the envelope, put the lab in a new envelope and send it to the arbiter, who then sends me the two drawn cards--he keeps the physical copy of Lab because I don't need it any longer. I then send my whole hand back with my buy orders.
I recommend only doing this within national borders, though, as air mail can be slower than you might like.
I'm often able to finish a game in as little as eighteen months, and for only around $80 in postage between the players. But unlike Goko, I can play several games at a time thus way! I'm about two years into a crazy Mountebank slog, but I'm pretty certain my opponent has a Mountebank in hand; just wait until he opens his mail a and sees the note that it was blocked by the Curse I'm holding!
Edit: if you really don't trust the mail, just use a courier as your arbiter. It will also be faster, as the arbiter/courier will be able to fly back and forth between you and your opponent, and you can take the courier out for coffee and try to get him to accidentally slip and tell you something about your opponent's hand!