Download the latest android package(aar file) from here https://mvnrepository.com/artifact/com.razorpay/checkout.
For more information on API usage refer this offcial Documentation: https://razorpay.com/docs/payment-gateway/android-integration/standard/
For step by step guide refer my blog:https://www.c-sharpcorner.com/article/creating-xamarin-android-binding/