App Checkout page and payment gateway integration

I'm looking for an experienced developer to create a checkout page where clients can choose between online payment or COD and integrate a payment gateway for my app. And after the payment confirmation page. The checkout page and payment gateway should be written in Kotlin, Java, and any other language chosen by the developer. The successful applicant should have a portfolio of past work and/or a detailed proposal detailing their experience and approach to the project. Regarding payment gateway integration, I am looking for either Razorpay or Cashfree.