Late Fee Limits?

I currently ran into a situation where a company owes me a rather large late fee settlement.

The original amount was $300, and i had a clause in the contract that stated a late fee of $25 which was signed.

The amount owed now is $1500. And the company finally wanted to pay. However it stated the maximum recoup amount in the state of Wisconsin is 18%. Which would come to around $650.

Two problems:

One they signed a contract which agreed to a $25/month late charge.

Two, what about payday loans? Don’t they charge like 200% interest?

see here: