JK Premises Coop Society, Sector 17, Vashi, Navi Mumbai
Excess penalty on delayed maintenance payment

I own an office in a commercial building. The society issues maintenance bill, quarterly (once in 3 months), in advance. The due date is set as middle of the quarter. Recently I paid the bill at the end of the quarter. The society has charged a penal interest of 21% on the whole bill amount starting from the 1 st day of the quarter, in the next bill! Is it right and legal?