Added an example for how to update a payment using payment.replace

Merge pull request #193 from pp-randy/master

Author: Randy

samples/payment/update_payment.py

@@ -0,0 +1,31 @@
+from paypalrestsdk import Payment
+import logging
+
+logging.basicConfig(level=logging.INFO)
+
+payment = Payment.find("<PAYMENT_ID>")
+
+# e.g. Update the amount, shipping address, invoice ID, or custom data after payment creation
+# You cannot update a payment after the payment executes.
+# https://developer.paypal.com/docs/api/payments/#payment_update
+update_payment_json = [
+    {
+        "op": "replace", 
+        "path": "/transactions/0/amount", 
+        "value": {
+            "total": "4.50", 
+            "currency": "USD",
+            "details": {
+                "subtotal": "5.00",
+                "shipping_discount": "-0.50"
+            }
+        }
+    }
+];
+
+
+if payment.replace(update_payment_json):
+    print("Payment[%s] updated successfully"%(payment.id))
+else:
+    print(payment.error)
+