Recurring billing - Marking new invoices as 'Paid'

[thisura]

Hello all,

I am currently developing a third party payment gateway plugin that supports recurring billing. I've been able to successfully post the recurring information of the products in a given invoice to my payment gateway.

The trouble I'm having is with the callback.

My payment gateway accepts a parameter called order_id (along with other parameters), to which I pass the invoice_id generated by WHMCS . I am able to extract this parameter from the callback file. Therefore, I am able to mark the initial invoice for a recurring payment as paid.

Now on the next recurring cycle, WHMCS generates a new invoice and my payment gateway sends a payment notification to the plugin's callback.

In this situation, I am unable to track the new invoice which was generated for the subscription, by WHMCS.

Is there a method built into WHMCS that I can use to track the new invoice? (for example via the subscription_id field, hooks, custom tables, etc.) If not, how is the PayPal plugin able to perform this kind of work?

Please also note that,

1.  I have the option of passing 2 custom parameters to the payment gateway which I can retrieve from the callback.
2.  I do receive a subscription_id unique to each subscription.

Any support would be greatly appreciated and please excuse me if the problem description above is not clear enough. I've described it to the best I can. Thank you in advance!