Full Code

BackendPost

using System.Collections.Generic;
using System.Text;
using UnityEngine;

// Add BACKND SDK namespace
using BackEnd;

public class Post
{
    public bool isCanReceive = false;

    public string title; // Mail title
    public string content; // Mail content
    public string inDate; // Mail inDate

    // 'string' is the name of the mail item and 'int' is the count
    public Dictionary<string, int> postReward = new Dictionary<string, int>();

    public override string ToString()
    {
        string result = string.Empty;
        result += $"title : {title}\n";
        result += $"content : {content}\n";
        result += $"inDate : {inDate}\n";

        if (isCanReceive)
        {
            result += "Mail Item\n";

            foreach (string itemKey in postReward.Keys)
            {
                result += $"| {itemKey} : {postReward[itemKey]}\n";
            }
        }
        else
        {
            result += "Unsupported mail item.";
        }

        return result;
    }
}

public class BackendPost
{
    private static BackendPost _instance = null;

    public static BackendPost Instance
    {
        get
        {
            if (_instance == null)
            {
                _instance = new BackendPost();
            }

            return _instance;
        }
    }

    private List<Post> _postList = new List<Post>();

    public void SavePostToLocal(LitJson.JsonData item)
    {
        foreach (LitJson.JsonData itemJson in item)
        {
            if (itemJson["item"].ContainsKey("itemType"))
            {
                int itemId = int.Parse(itemJson["item"]["itemId"].ToString());
                string itemType = itemJson["item"]["itemType"].ToString();
                string itemName = itemJson["item"]["itemName"].ToString();
                int itemCount = int.Parse(itemJson["itemCount"].ToString());

                if (BackendGameData.userData.inventory.ContainsKey(itemName))
                {
                    BackendGameData.userData.inventory[itemName] += itemCount;
                }
                else
                {
                    BackendGameData.userData.inventory.Add(itemName, itemCount);
                }

                Debug.Log($"Item received. : {itemName} - {itemCount}");
            }
            else
            {
                Debug.LogError("Unsupported item.");
            }
        }
    }

    public void PostListGet(PostType postType)
    {
        var bro = Backend.UPost.GetPostList(postType);

        if (bro.IsSuccess() == false)
        {
            Debug.LogError("An error occurred while loading the mail.");
            return;
        }

        Debug.Log("Successfully requested mail list to be loaded. : " + bro);

        if (bro.GetFlattenJSON()["postList"].Count <= 0)
        {
            Debug.LogWarning("There is no mail to receive.");
            return;
        }

        foreach (LitJson.JsonData postListJson in bro.GetFlattenJSON()["postList"])
        {
            Post post = new Post();

            post.title = postListJson["title"].ToString();
            post.content = postListJson["content"].ToString();
            post.inDate = postListJson["inDate"].ToString();

            // Item in the mail
            if (postType == PostType.User)
            {
                if (postListJson["itemLocation"]["tableName"].ToString() == "USER_DATA")
                {
                    if (postListJson["itemLocation"]["column"].ToString() == "inventory")
                    {
                        foreach (string itemKey in postListJson["item"].Keys)
                        {
                            post.postReward.Add(itemKey, int.Parse(postListJson["item"][itemKey].ToString()));
                        }
                    }
                    else
                    {
                        Debug.LogWarning("This column information is not supported yet. : " +
                                         postListJson["itemLocation"]["column"].ToString());
                    }
                }
                else
                {
                    Debug.LogWarning("This table information is not supported yet. : " +
                                     postListJson["itemLocation"]["tableName"].ToString());
                }
            }
            else
            {
                foreach (LitJson.JsonData itemJson in postListJson["items"])
                {
                    if (itemJson["chartName"].ToString() == chartName)
                    {
                        string itemName = itemJson["item"]["itemName"].ToString();
                        int itemCount = int.Parse(itemJson["itemCount"].ToString());

                        if (post.postReward.ContainsKey(itemName))
                        {
                            post.postReward[itemName] += itemCount;
                        }
                        else
                        {
                            post.postReward.Add(itemName, itemCount);
                        }

                        post.isCanReceive = true;
                    }
                    else
                    {
                        Debug.LogWarning("This chart information is not supported yet. : " + itemJson["chartName"].ToString());
                        post.isCanReceive = false;
                    }
                }
            }

            _postList.Add(post);
        }

        for (int i = 0; i < _postList.Count; i++)
        {
            Debug.Log($"Mail No. {i}\n" + _postList[i].ToString());
        }
    }

    public void PostReceive(PostType postType, int index)
    {
        if (_postList.Count <= 0)
        {
            Debug.LogWarning("There is no mail to receive. Or, call the mail list loading first.");
            return;
        }

        if (index >= _postList.Count)
        {
            Debug.LogError($"The mail does not exist. : Request index{index} / maximum mail count : {_postList.Count}");
            return;
        }

        Debug.Log($"{_postList[index].inDate} of {postType.ToString()} Requesting mail claim.");

        var bro = Backend.UPost.ReceivePostItem(postType, _postList[index].inDate);

        if (bro.IsSuccess() == false)
        {
            Debug.LogError($"{_postList[index].inDate} of {postType.ToString()} An error occurred while claiming the mail. : " + bro);
            return;
        }

        Debug.Log($"{_postList[index].inDate} of {postType.ToString()} Successfully claimed mail. : " + bro);

        _postList.RemoveAt(index);

        if (bro.GetFlattenJSON()["postItems"].Count > 0)
        {
            SavePostToLocal(bro.GetFlattenJSON()["postItems"]);
        }
        else
        {
            Debug.LogWarning("There is no mail item to be claimed.");
        }

        BackendGameData.Instance.GameDataUpdate();
    }

    public void PostReceiveAll(PostType postType)
    {
        if (_postList.Count <= 0)
        {
            Debug.LogWarning("There is no mail to receive. Or, call the mail list loading first.");
            return;
        }

        Debug.Log($"{postType.ToString()} Requesting all pieces of mail to be claimed.");

        var bro = Backend.UPost.ReceivePostItemAll(postType);

        if (bro.IsSuccess() == false)
        {
            Debug.LogError($"{postType.ToString()} An error occurred while claiming all mail : " + bro);
            return;
        }

        Debug.Log("Successfully claimed all mail. : " + bro);

        _postList.Clear();

        foreach (LitJson.JsonData postItemsJson in bro.GetFlattenJSON()["postItems"])
        {
            SavePostToLocal(postItemsJson);
        }

        BackendGameData.Instance.GameDataUpdate();
    }
}

BackendManager.cs

using UnityEngine;


// Add BACKND SDK namespace
using BackEnd;

public class BackendManager : MonoBehaviour
{
    void Start()
    {
        var bro = Backend.Initialize(); // Initialize BACKND

        // Response value for BACKND initialization
        if (bro.IsSuccess())
        {
            Debug.Log("Initialization successful : " + bro); // If successful, 'statusCode 204 Success'
        }
        else
        {
            Debug.LogError("Initialization failed : " + bro); // If failed, a 4xx statusCode error occurs 
        }

        Test();
    }

    // A method that allows synchronous methods to be called from asynchronous methods (cannot be accessed by the Unity UI)
    void Test()
    {
        BackendLogin.Instance.CustomLogin("user1", "1234");

        // Load game data and save it locally. (caching)
        BackendGameData.Instance.GameDataGet();

        // Load the mail list and save the inDate values locally.  
        BackendPost.Instance.PostListGet(PostType.Admin);

        // Claim the mail by reading the position of the saved piece of mail. Here, the index is the order of the mail; 0 is the topmost and 1 is the next.
        BackendPost.Instance.PostReceive(PostType.Admin, 0);

        // Claim all pieces of mail that have been looked up.  
        BackendPost.Instance.PostReceiveAll(PostType.Admin);

        Debug.Log("Test complete.");
    }
}

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