Medlife — already paid online but asked for payment on delivery

I ordered 2 things from medlife and for which i already paid Rs.307.2 through paytm. And when it was delivered they asked for the money Rs 310.And now i want refund of Rs.310 for the order for which i already paid.And i also want to know through which mode of payment money will be refunded.
Order Id:OR2378HUA-002A
Serial Number:AMD[protected]
CIN-U24230BR2014PTC023149
Date of invoice:04/09/19