Here are three examples for the Payment Method, Payment, and Refund objects.

Payment Method object

Request 
{
    "billingAccount": {
        "accountNumber": "A-1621965929897",
        "currency": "USD"
    },
    "operation": "Validate",
    "paymentGatewayName": "UPCTokenGateway1621965929867",
    "paymentMethod": {
        "authAmount": "1",
        "currency": "USD",
        "type": "AmazonPay__c_12368",
        "upcTokenData": {
            "AmazonAccount": "SampleAmazonAccount",
            "AmazonToken": "3sample54cf04113e3f1595951874003",
            "AmazonTokenType": "Digital",
            "ShopperEmail": "sample@testmail.com",
            "ShoppingDate": ""
        }
    }, 
    "tenantId": "12368"
}
Response (HTTP code: 200)
{
    "gatewayResponseCode": "601",
    "gatewayResponseMessage": "The transaction has been approved.",
    "gatewaySecondTransactionId": "735788334",
    "gatewayTransactionId": "383315250",
    "responseCode": "Approved",
    "upcTokenData": "{ \"ShopperEmail\": \"sample@testmail.com\"}"
}

Payment object

Request
{
    "billingAccount": {
        "accountNumber": "A00000004",
        "currency": "USD"
    },
    "operation": "Payment",
    "payment": {
        "amount": "200",
        "currency": "USD",
        "id": "4028818579a43c3f0179aba917410419",
        "paymentNumber": "P-00000011"
    },
    "paymentGatewayName": "UPC_Token",
    "paymentMethod": {
        "id": "4028818579a43c3f0179aba2808103e8",
        "type": "AmazonPay__c_12368",
        "upcTokenData": {
            "AmazonAccount": "SampleAmazonAccount",
            "AmazonToken": "3sample54cf04113e3f1595951874003",
            "AmazonTokenType": "Digital",
            "ShopperEmail": "sample@testmail.com",
            "ShoppingDate": ""
        }
    },
    "tenantId": "12368"
}
Response  (HTTP code: 200)
{
    "gatewayResponseCode": "601",
    "gatewayResponseMessage": "The transaction has been approved.",
    "gatewaySecondTransactionId": "20998810",
    "gatewayTransactionId": "180404672",
    "responseCode": "Approved",
    "upcTokenData": "{ \"ShopperEmail\": \"sample@testmail.com\"}"
}

Refund object

Request 
{
    "billingAccount": {
        "accountNumber": "A00000004",
        "currency": "USD"
    },
    "operation": "Refund",
    "paymentGatewayName": "UPC_Token",
    "paymentMethod": {
        "id": "4028818579a43c3f0179aba2808103e8",
        "type": "AmazonPay__c_12368",
        "upcTokenData": {
            "AmazonAccount": "SampleAmazonAccount",
            "AmazonToken": "3sample54cf04113e3f1595951874003",
            "AmazonTokenType": "Digital",
            "ShopperEmail": "sample@testmail.com",
            "ShoppingDate": ""
        }
    },
    "refund": {
        "amount": "200",
        "id": "4028818579a43c3f0179aba6ebfb040d",
        "paymentId": "4028818579a43c3f0179aba2b14a03ea",
        "referenceId": "166435652",
        "refundNumber": "R-00000005"
    },
    "tenantId": "12368"
}
Response (HTTP code: 200)
{
    "gatewayResponseCode": "601",
    "gatewayResponseMessage": "The transaction has been approved.",
    "gatewaySecondTransactionId": "687106060",
    "gatewayTransactionId": "760690295",
    "responseCode": "Approved"
}

Related information

Overview of integrating your custom payment gateways and payment methods