Must be some sorta rule...

Translation companies that threaten draconian penalties for late submittal of work you owe them feel free to blithely ignore the "net 30 days" terms stated in your invoice for work submitted on time.

Le "sigh."

UPDATE: The company that prompted this post responded to my (second) email, informing me, among other things, that the check was going in the mail today, and that their payment terms were "net 45 days" (so that technically, they were paying me "early").