question

@Ivan Cai

The code didn't fit in the comment, so I'm posting here as an answer.

EDIT: This issue happened to only one user. We haven't had this issue before.

handlers.BuyContainer = function(args, context) {
    if(args == null || args.ItemID == null || args.BuyAmount == null) return 0;
    if(args.ItemID === "" || args.BuyAmount === "" || isNaN(args.BuyAmount)) return 1;

    // GetItemByID returns the related item from the catalog
    var item = GetItemByID(args.ItemID);
    if(item == null)
        return 2;

    if(item.ItemClass != "LootBox")
        return 3;

    var itemCustomData = JSON.parse(item.CustomData);
    if(parseInt(itemCustomData["Buyable"]) != 1)
        return 4;

    var amount = parseInt(args.BuyAmount);
    // GetInstanceByID returns the stackable item in the player's inventory if there is any. If not, it returns null
    var instance = GetInstanceByID(currentPlayerId, args.ItemID);
    if(instance != null) {
        var total = instance.RemainingUses != null ? parseInt(instance.RemainingUses) : 1;
        if(total + amount > 200)
            return 5;
    }
    else if(amount > 200)
        return 5;

    var getDiscount = {
        "Keys": ["DiscountContainer"]
    };
    var getDiscountResult = server.GetTitleData(getDiscount);

    var price = parseInt(item.VirtualCurrencyPrices["CO"]);
    var discount = price * (parseFloat(getDiscountResult.Data["DiscountContainer"]) / 100.0);
    var finalPrice = (Math.floor(price - discount) * amount) + 1;

    var getCoins = {
        "PlayFabId": currentPlayerId,
        "Amount": 1,
        "VirtualCurrency": "CO"
    };
    var getCoinsResult = server.AddUserVirtualCurrency(getCoins);
    var myCoins = parseInt(getCoinsResult.Balance);

    if(myCoins < finalPrice) {
        var removeCoins = {
            "PlayFabId": currentPlayerId,
            "Amount": 1,
            "VirtualCurrency": "CO"
        };
        server.SubtractUserVirtualCurrency(removeCoins);

        return { "Coins": (myCoins - 1) };
    }
    
    var decreaseCoins = {
        "PlayFabId": currentPlayerId,
        "Amount": finalPrice,
        "VirtualCurrency": "CO"
    };
    server.SubtractUserVirtualCurrency(decreaseCoins);

    if(instance == null) {
        var newContainer = {
            "PlayFabId": currentPlayerId,
            "CatalogVersion": "Items",
            "ItemIds": [item.ItemId]
        };
        var newContainerResult = server.GrantItemsToUser(newContainer);

        amount--;
        var addInstance = {
            "PlayFabId": currentPlayerId,
            "ItemInstanceId": newContainerResult.ItemGrantResults[0].ItemInstanceId,
            "UsesToAdd": amount
        }
        server.ModifyItemUses(addInstance);

        return { "InstanceID": newContainerResult.ItemGrantResults[0].ItemInstanceId };
    }
    else {
        var addInstance = {
            "PlayFabId": currentPlayerId,
            "ItemInstanceId": instance.ItemInstanceId,
            "UsesToAdd": amount
        }
        server.ModifyItemUses(addInstance);

        return { "InstanceID": instance.ItemInstanceId };
    }
};