Hi.
It looks like you have issue the same as for pp_standart.
After pressing button "Confirm" on checkout/conform page your customer will be reditected to third-party site (in your case easyBuzz side).
Then, after payment, will be redirected back on checkout/success page. But connection server-server (i mean post-request from their API to your response/callback) will be after redirect. In this case checkout/success see that order status is zero (incomplete) and set it to "failed".
Please check our solution for paypal_standart extension. We created additional page with periodical ajax and asks response controller is payment confirmed (page for pending of api confirmation).
Logic of this:
1. create hook to checkout/success controller InitData() and check is order incomplete and payment method is easyBuzz. If true - redirect to "pending payment page".
2. This page asks every 5 seconds your response controller method is_confirmed() with timeout 30sec. If order status still 0 - returns false. if status greater 0(api asked callback and changed order status) - returns true. If "true" OR timeout exceed - redirect back to checkout/success page. In 1st case order will be completed correctly. Otherwise - failed
Please check this commit
https://github.com/abantecart/abantecart-src/commit/0a5adf666a869393ee30f1d38689fb09b6fd1cbeYou can copy whole file with hooks, methods pending_payments() and is_confirmed(), and pending_ipn.tpl
Hope this hepls