Allow the client to override CRM.payment.getTotalAmount() function

js/crm.payment.js

@@ -157,6 +157,10 @@
     CRM.payment = payment;
   }
   else {
+    if (CRM.payment.hasOwnProperty('getTotalAmount')) {
+      delete(payment.getTotalAmount);
+      payment.debugging(payment.scriptName, 'Deferring to client getTotalAmount function');
+    }
     $.extend(CRM.payment, payment);
   }