add rate service

lib/domain/entities/custom_duty.dart

@@ -0,0 +1,25 @@
+class CustomDuty {
+  String id;
+  String productType;
+  String desc;
+  double fee;
+  CustomDuty({this.id, this.productType, this.desc, this.fee});
+
+  factory CustomDuty.fromMap(Map<String, dynamic> map, String id) {
+    return CustomDuty(
+      id: id,
+      productType: map['product_type'],
+      desc: map['desc'],
+      fee: (map['fee'] ?? 0).toDouble(),
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      "id": id,
+      'product_type': productType,
+      'desc': desc,
+      'fee': fee,
+    };
+  }
+}