Added a bit about using some form of package tracking if you mail in a payment to http://www.livejournal.com/support/faqbrowse.bml?faqid=21
The Bit
We strongly suggest using a carrier or delivery method with delivery or signature confirmation as we have no way of tracking a payment that hasn't reached our offices. Also, please make sure you allow plenty of time for us to receive your payment, since we can't offer account extensions due to delayed mail delivery.
Ryan is also going to add the same bit to the invoice page.