add paginator

lib/domain/entities/rate.dart

@@ -12,6 +12,12 @@ class Rate {
   List<CustomDuty> customDuties;
   List<DiscountByWeight> discountByWeights;
 
+  DiscountByWeight getDiscountByWeight(double weight) {
+    discountByWeights.sort((d1, d2) => d2.weight.compareTo(d1.weight));
+    return discountByWeights.firstWhere((e) => e.weight < weight,
+        orElse: () => null);
+  }
+
   CargoType get defaultCargoType => cargoTypes == null
       ? null
       : cargoTypes.firstWhere((e) => e.name == "General");