We are running a startup where we connect the consumer to the nearest labour. But the major difference is that we have no information about the amount that the plumber, carpenter etc. charges from the customer. We only charge a fixed monthly membership fee from the labour, plumber etc. Are we liable to pay GST u/s 9(5)? If yes then how will we calculate the amount of GST to be paid? Also is the cenvat credit of the tax so paid available?