You forgot to attach your page.
I am keen to help you resolve your problem, but don't want to burden you with stuff you aren't comfortable with.
To see if the problem is one of validation, you should have some sever validation code on your page, or on the redirected page after posting
If it is there, you can comment out the validation for zip code, and if the page works ok, then this looks like the problem.
the line will look something like...
$WAFV_Errors .= WAValidateZC(((isset($_POST["postcode"]))?$_POST["postcode"]:"") .....etc);
If this is the problem, I can help you use the modified function to get the validation working