On a much smaller scale I did the same thing with a consulting contract. They sent it me and said to full in my own job description and "check the contract". The laziness annoyed me, so I altered the payment terms from 30 days to 7. Every month they paid after around 15 days and I let it go, but one month they hadn't paid after 31 days and I sent them an invoice for late payment for every single invoice to that date (only 4 or 5). I didn't think they'd pay it but they did

Now this is a case that's something I can get behind and fight for.