Legal Question in Business Law in California

I’m afraid an error on the invoice does not modify the terms of your contract. Personally as good customer service I would never enforce a late fee in a contract when I made an error on the invoice and misled my customer into paying late, but legally the contract controls the rights of the parties.

That said, however, late fees must be reasonable. The law treats them as liquidated damages. That means they must be fair compensation for damages the creditor suffers due to receiving payment late, but that would be too hard or costly to prove exactly. Excessive late fees are considered penalties which cannot be legally imposed or enforced. So depending on the size of the fee compared to the total of the invoice, you may have bargaining room there, especially if you have other vendor options and they want to keep you as a customer.