fix number formatting

lib/domain/entities/cargo_type.dart

@@ -1,5 +1,3 @@
-import 'dart:convert';
-
 class CargoType {
   String id;
   String name;

lib/domain/entities/invoice.dart

@@ -38,7 +38,7 @@ class Invoice {
           CargoType existing = cargoTypes.firstWhere((wc) => wc.id == tc.id);
           existing.weight += tc.weight;
         } else {
-          cargoTypes.add(tc);
+          cargoTypes.add(tc.clone());
         }
         actualWeight += tc.weight;
       });