fix: correct usage of line items shipment property (closes #645) (#646)

# Description

See #645 for more details, this corrects it.

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# Testing

Added `line_items` to a shipment in a test and it successfully passed
the params with this change.

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# Pull Request Type

Please select the option(s) that are relevant to this PR.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Improvement (fixing a typo, updating readme, renaming a variable
name, etc)

Author: Justintime50

EasyPost/Models/API/LineItem.cs

@@ -0,0 +1,27 @@
+using EasyPost._base;
+using Newtonsoft.Json;
+
+namespace EasyPost.Models.API
+{
+    /// <summary>
+    ///     Class representing line items of a <a href="https://docs.easypost.com/docs/shipments">Shipment</a>.
+    /// </summary>
+    public class LineItem : EasyPostObject, Parameters.ILineItemParameter
+    {
+        #region JSON Properties
+
+        /// <summary>
+        ///     The total value of the line item.
+        /// </summary>
+        [JsonProperty("total_line_value")]
+        public string? TotalLineValue { get; set; }
+
+        /// <summary>
+        ///     The description of the item.
+        /// </summary>
+        [JsonProperty("item_description")]
+        public string? ItemDescription { get; set; }
+
+        #endregion
+    }
+}

EasyPost/Models/API/Shipment.cs

@@ -77,6 +77,12 @@ public class Shipment : EasyPostObject, Parameters.IShipmentParameter
         [JsonProperty("is_return")]
         public bool? IsReturn { get; set; }
 
+        /// <summary>
+        ///     A list of <see cref="LineItem"/>s associated with the shipment.
+        /// </summary>
+        [JsonProperty("line_items")]
+        public List<LineItem>? LineItems { get; set; }
+
         /// <summary>
         ///     A list of any carrier errors that occurred during rating or purchasing.
         /// </summary>

EasyPost/Parameters/IParameter.cs

@@ -63,6 +63,13 @@ public interface IInsuranceParameter : IParameter
     {
     }
 
+    /// <summary>
+    ///     An interface marking that an instance of the implementing class can be used as a line item parameter in a Parameters object.
+    /// </summary>
+    public interface ILineItemParameter : IParameter
+    {
+    }
+
     /// <summary>
     ///     An interface marking that an instance of the implementing class can be used as an order parameter in a Parameters object.
     /// </summary>

EasyPost/Parameters/Shipment/Create.cs

@@ -155,27 +155,11 @@ public class Create : BaseParameters<Models.API.Shipment>, IShipmentParameter
         public List<string>? CarrierAccountIds { get; set; }
 
         /// <summary>
-        ///     A list of line items for the new <see cref="Models.API.Shipment"/>.
+        ///     A list of <see cref="Models.API.LineItem"/>s for the new <see cref="Models.API.Shipment"/>.
         /// </summary>
         [TopLevelRequestParameter(Necessity.Optional, "shipment", "line_items")]
         [NestedRequestParameter(typeof(Order.Create), Necessity.Optional, "line_items")]
-        public List<LineItem>? LineItems { get; set; }
-
-        /// <summary>
-        ///     Represents a line item for a shipment.
-        /// </summary>
-        public class LineItem
-        {
-            /// <summary>
-            ///     The total value of the line item.
-            /// </summary>
-            public string? TotalLineValue { get; set; }
-
-            /// <summary>
-            ///     The description of the item.
-            /// </summary>
-            public string? ItemDescription { get; set; }
-        }
+        public List<ILineItemParameter>? LineItems { get; set; }
 
         #endregion
     }