Contract law works under elements of reasonable treatment and many clauses can be challenged. This would be one.
I was toying with the idea of stating that this may possibly be an unreasonable condition, but decided against it because -
1. The agency controls the money and has exclusive knowledge of sales, so the plaintiff is on the back-foot already.
2. The agency could easily claim that they do not earn anything on sales at under $50.
As for being snooty (I assume that was directed at me!) I have just written a book about business mistakes and the real reason businesses fail (coming out later in the year). I did not dream to include 'not reading contracts' as this is such a fundamental rule of life, that doing such a thing is like insisting on driving with your eyes closed.
I do try to go out of my way to help my fellow human beings, but I throw in the towel and walk away if they don't deal with issues immediately, insist on repeating their mistakes and sign up for things without realising the ramifications.