...make an online service that sends a validation code by mail.

Not email, real mail. You know the kind with stamps, and ponies, and Kevin Costner.

ReachServices* allow you to look at your tax record and apply for refunds online. As anyone who has ever been the tax office in Cathedral St can testify, this would be pretty handy. However the catch is they have to confirm your PPS number, and they only way they seem to be able to do this is to send a validation code to your postal address. To avoid making things too inconvenient they will send you a reminder mail around the time the letter should have arrived prompting you to enter your code. The email also warns you that it will expire soon. 'Soon', when is 'soon'? What if I don't get the letter before the code expires? Will they send me another? Will they use exponential backoff or are all of us in the boglands doomed?

* Is that an aging Gary Lineker posing in their business clip art?