Shall I go old school and appeal by post? (recorded delivery?)
No - do it online, it's free!
Also, if you ever send anything about these sort of charges by post,
don't do it recorded. If the recipient declines to sign for the delivery, or it doesn't arrive for any other reason, all you're left with is proof your letter was
not delivered. If you send it standard first class with a free certificate of posting, you enjoy a presumption of delivery.
But, in general, if you can do things online or by email it's sensible to do so.