Interest charged on delay

Hello Sir, i am taking flat under mhada scheme, wherein Cost of flat is Rs. 835772/- I have made payment 1st installment : Rs. 208492 (25% of flat cost) while registration 2nd Installment :Rs. 627588/- via bank loan. Now the builder have asked to pay following: Balance cost of flat : Rs. 32532/ (as they have deducted late payment intestest from 2nd installment i have paid) Society Charges : Rs. 86151/- GST on maintainance : Rs. 10890/- Total : 129574/- The builder has charged me interest as follows : 1. Rs. 308/- delay in 1st installment (for 5 days) 2. Rs. 12506/- delay in 2nd installment (for 68 days) 3. Rs. 20000/- as delay payment charges as 2nd installment is delayed more than 60 days So total Rs. 32841/- (late payment intestest of Rs. 12841/- and Rs. 20000/- delay payment charges). When spoken to them, they have told me to make payment of rs. 109574/- as final due (they are waiving off Rs. 20000/-). Builder have sent me the receipt of payment made by me but no documents have been provide with regards to final due or interest charged. On phone they have given above details. Please guide : 1. Whether builder can charge such huge interest. 2. whether GST on maintainance to be paid. Please help..