Payment Gateway Integration error solving - asp.net

Question

I have a windows application that requires Payment Gateway Integration.
All these stuff i have done.
The process includes the following...
-Creating Xml Request header
-Encoding it as Url
-send the url encoded data to a Rest API with link being provided in the documentation.
-The test server performs the transaction and sends response back to the application.
What I have done to achieve these functionality...
1) I have used 'HttpWebRequest' class to create request to the URL : I am not going to share it since it security and confidential.
2) I have generated xml header and send it as url encoded.
3) The Server is even responses back to the application. but with Status not success
I am going to post the request formate and response formate.
<?xml version="1.0" encoding="UTF-8"?>
<payment xmlns="http://www.elastic-payments.com/schema/payment">
<merchant-account-id>!#-merchant-account-id-#!</merchant-account-id>
<request-id>!#-request-id-#!</request-id>
<transaction-type>purchase</transaction-type>
<requested-amount currency="AUD">!#-requested-amount-#!</requested-amount>
<account-holder>
<first-name>!#-first-name-#!</first-name>
<last-name>!#-last-name-#!</last-name>
<email>!#-email-#!</email>
<phone>!#-phone-#!</phone>
<address>
<street1>!#-street1-#!</street1>
<street2>!#-street2-#!</street2>
<city>!#-city-#!</city>
<state>!#-state-#!</state>
<country>!#-country-#!</country>
<postal-code>!#-postal-code-#!</postal-code>
</address>
</account-holder>
<card>
<account-number>!#-account-number-#!</account-number>
<expiration-month>!#-expiration-month-#!</expiration-month>
<expiration-year>!#-expiration-year-#!</expiration-year>
<card-type>!#-card-type-#!</card-type>
<card-security-code>!#-card-security-code-#!</card-security-code>
</card>
<notifications>
<notification url="!#-notification-url-#!" transaction-state="failed" />
<notification url="!#-notification-mail-#!" />
</notifications>
<custom-fields>
<custom-field field-name="order no" field-value="!#-order no-#!" />
</custom-fields>
<ip-address>!#-ip-address-#!</ip-address>
</payment>
The following is the response coming back from the server.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<payment xmlns="http://www.elastic-payments.com/schema/payment" self="http://sandbox-engine.thesolution.com/engine/rest/merchants/null/payments/null">
<merchant-account-id ref="http://sandbox-engine.thesolution.com/engine/rest/config/merchants/null"/>
<transaction-state>failed</transaction-state>
<completion-time-stamp>2012-11-28T08:19:31.024Z</completion-time-stamp>
<statuses>
<status code="400.1018" description="The same Request Id for the Merchant Account is being tried a second time. Please use another Request Id. " severity="error"/>
<status code="400.1009" description="The Transaction Type is invalid. Please check your input and try again." severity="error"/>
</statuses>
<requested-amount/>
<api-id>elastic-api</api-id>
</payment>
I have cross checked twice with the request header,Each time i do have unique Request id which is 'hiren-'+GUID+DATETIMEWITHTIME
I even have cross checked with the documentation of the gateway for the transaction-type and it's valid but even then it's giving the same error.
To encode url i have used the following code
string sendString = "param=" + HttpUtility.UrlEncode(xmlData);
I'm thinking that the above code may be having the error but it's giving reply back...
so need help to solve the problem.
Please don't ask for the document file but you can ask for extra information.