There are laws against excessive fees like this. In my home state of Florida, a landlord cannot charge more than 20% the rent in late fees.

And trust me, Florida is not known for its consumer protections. So chances are your state has a similar law which is probably better.

So if I were you I would look up the law and not pay.

Assuming of course you are telling the full truth- your full rent payment was late for a short period of time.