Also because you are wanting the feature to only work while accessing FROM the admin as in while logged into the admin and making changes there, then if you test (IS_ADMIN_FLAG === true) is a condition where the admin (system not a "person") is accessing the payment module from the admin side.
Otherwise, if you know your admin's customer_id, then you could check if that customer is accessing the payment module:
Code:
if (isset($_SESSION['customer_id']) && $_SESSION['customer_id'] == 15) {
// enable
}
Where 15 is the customer id for your "admin user" logged in on the store side.
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