Full Code

BackendPost

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

// Adds 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

    // The string is the name of the mail item and the 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]}ea\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}ea");
            } 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() == "Item Chart") {
                        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;
using System.Threading.Tasks;

// Adds BACKND SDK namespace
using BackEnd;

public class BackendManager : MonoBehaviour {
    void Start() {
        var bro = Backend.Initialize(true); // 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)
    async void Test() {
        await Task.Run(() => {
            BackendLogin.Instance.CustomLogin("user1", "1234");

            // Loads game data and saves it locally.(caching)
            BackendGameData.Instance.GameDataGet();

            // Loads the mail list and saves the inDate values locally.  
            BackendPost.Instance.PostListGet(PostType.Admin);
            
            // Claims 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);
            
            // Claims all pieces of mail that have been looked up.  
            BackendPost.Instance.PostReceiveAll(PostType.Admin);
            
            Debug.Log("Test complete.");
        });
    }
}

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