Fixes #3349. Thanks @abelbm for reporting.
diff --git a/erpnext/controllers/selling_controller.py b/erpnext/controllers/selling_controller.py
index 2dde685..9a2da96 100644
--- a/erpnext/controllers/selling_controller.py
+++ b/erpnext/controllers/selling_controller.py
@@ -85,9 +85,9 @@
 			existing_shipping_charge = self.get("taxes", filters=shipping_charge)
 			if existing_shipping_charge:
 				# take the last record found
-				existing_shipping_charge[-1].rate = shipping_amount
+				existing_shipping_charge[-1].tax_amount = shipping_amount
 			else:
-				shipping_charge["rate"] = shipping_amount
+				shipping_charge["tax_amount"] = shipping_amount
 				shipping_charge["description"] = shipping_rule.label
 				self.append("taxes", shipping_charge)