We set up an account for automatic payments. Works fine when you are using the crossing regularly. However, if you don't use the crossing for a year, they close your account and its up to you to phone them to get the balance left (in my case just shy of £20).I missed payment one time many years ago and remembered days later. I phoned them to grovel and to my astonishment a human answered the call and said don't worry just go online and pay it now, you won't get a letter.
I did use the crossing a few days after they closed the account, and received a nice letter claiming a penalty charge. Reference number was my (closed) account...
BTW, first offence you just pay the standard crossing fee if you contact them quickly.