I'm a freelance UX designer in Massachusetts. I'm in the process of collecting payment from a client. In our contract, it states "[...] payable in arrears no later than 10 days after the invoice date." In my invoice, it states "If this invoice is unpaid by Jul 12, 2020, a non-compounding late fee of 10.0% accrues monthly on the outstanding amount." They raised no issues and paid the contract amount, but not the late fee. When inquiring on the late fee, they stated that my request is illegal by MA standards, saying: "[...] charging 10% late fee for 3 days is illegal in the state of MA. Besides the point, (Company Name) has a flat Net 30 on all payments and does not pay anything beyond 12% ANNUALLY for late payments." Looking for clarity on this. Thank you!