I'm sympathetic to the idea that requiring prompt payment for work done is a reasonable thing. But ten days doesn't work in (most of) the real world.
For many people in a position of hiring contractors to do work, there are times when an invoice can sit in an inbox -- physical or otherwise -- for more time than that before even being opened, much less acted upon via a process that often involves multiple other people.
Most companies I've experienced would respond to a demand for a 50% late fee by paying the exactly the original amount, exactly 30 days after invoice, and simply never work with you again.
He says right on top that thats negotiable _upfront_. I usually have net 15 and am pretty strict on that. That means, I will call on that day and ask for the invoice to be paid. I also make it clear to clients that prolonged non-payment outside of terms will lead to a raise on the negotiation. I also make clear that if they cannot pay for whatever reason, they can just call and talk about things. The reason why the default number is low is pretty easy: clients will only go up from there.
I personally find the punishment extremely hard. 10% per week is definitely unreasonable. It would basically put that person on my "don't recommend that colleague"-list.
About companies not paying late fees: unless the contract is really beefy to start with, its also a quick way to ensure that good freelancers won't work with you again. Word goes around. I've seen more that one seemingly "big" company struggle with not having any business partners to turn to any more.
For many people in a position of hiring contractors to do work, there are times when an invoice can sit in an inbox -- physical or otherwise -- for more time than that before even being opened, much less acted upon via a process that often involves multiple other people.
Most companies I've experienced would respond to a demand for a 50% late fee by paying the exactly the original amount, exactly 30 days after invoice, and simply never work with you again.