Thanks for contacting us! Sorry about that, I have yet to put up a FAQ about dealing with that problem. We have found in the past to deal with problems like that you just add a N to the end of your post code, it signifies it as a blank space and accepts the payment. It has worked for all other countries so far, but this is new to us so please give us an email back to tell us whether or not it worked, we would really appreciate the feedback.
Usability experts would be turning in their graves... if only they were dead...
Your reply tells me that you have no access to the people who develop the interface, and therefore can't get tweaks made to fix usability problems. The solution is not changing user behaviour, but changing the behaviour of the web based interface itself. A simple one-time change to the error checking within the Javascript will mean you won't have to add a FAQ entry or deal with user feedback multiple times. Besides, some users wouldn't think to look in the FAQ and might instead just give up trying to make a pledge.