Grant membership subscription only after successful payment

Hi there,

I was hoping if someone would be able to help me. I want users to be able to sign up for my blog only after successful payment is made. Here is the code provided by my local payment gateway (called eSewa):

<form action = “https://dev.esewa.com.np/epay/main” method=”POST”> <input value=”100” name=”tAmt” type=”hidden”>
<input value=”90” name=”amt” type=”hidden”>
<input value=”5” name=”txAmt” type=”hidden”>
<input value=”2” name=”psc” type=”hidden”>
<input value=”3” name=”pdc” type=”hidden”>
<input value=”testmerchant” name=”scd” type=”hidden”>
<input value=”XYZ-1234” name=”pid” type=”hidden”>
<input value=”https://abc.com/success.html?q=su” type=”hidden” name=”su”> <input value=”https://abc.com/failure.html?q=fu” type=”hidden” name=”fu”> <input value=”Submit” type=”submit”>
</form>

Once the payment is successful, the gateway sends a response URI:

https://abc.com/success?q=su&oid=XYZ1234&amt=100&refId=000AE01

I havent been able to figure out how to link this success URI to granting subscription to the user. Some help will be highly appreciated!!