Interestingly, I used Free Filable Forms 2 years ago, but came up with the same post code issue last year. I emailed via the link provided, and got a standard answer telling me to follow the instructions (something I had already tried...if the agent had READ the email before replying, he would have known this). So tried again to no avail.
I ended up using the forms to fill out the return, printed with the post code blank, filled in with a pen, and mailed the return. I was visiting family in the US in March anyway, so just took it along and sent from there. We had no US tax due, and it was a simple filing.