question

Here's my Cloudscript implementation of SethDu's first bullet. (Although it does not meet the requirements of the OP's request to purchase multiple of the same product.)

handlers.PurchaseItems = function (args)
{
    const itemListArray = args.itemList;
    const itemListSet = new Set(itemListArray);
    const clientPrice = args.price;
    
    // 1. get all catalog items
    let catalog;
    try {
        catalog = server.GetCatalogItems({}).Catalog;
    } catch(e) {
        return 500;
    }
    
    // 2. for each passed item, total the prices from the catalog
    let serverPrice = 0;
    
    for (let catalogItemIndex = 0; catalogItemIndex < catalog.length; catalogItemIndex++)
    {
        if (itemListSet.has(catalog[catalogItemIndex].ItemId))
        {
            serverPrice += catalog[catalogItemIndex].VirtualCurrencyPrices.yourVirtualCurrency;
        }
    }
    
    // 3. compare the passed price to the actual price
    if (serverPrice != clientPrice)
    {
        return 1053;
    }
    
    // 4. make sure the player has enough virtualCurrency
    const getPlayerCombinedInfoRequest = {
        "PlayFabId": currentPlayerId,
        "InfoRequestParameters": {
            "GetUserVirtualCurrency": true
        }
    };
    
    let virtualCurrencyBalance;
    
    try {
        virtualCurrencyBalance = server.GetPlayerCombinedInfo(getPlayerCombinedInfoRequest).InfoResultPayload.UserVirtualCurrency.yourVirtualCurrency;
    } catch(e) {
        return 500;
    }
    
    if (virtualCurrencyBalance < serverPrice)
    {
        return 1059;
    }


    // 5. grant items
    const grantItemsToUserRequest = {
        "ItemIds": itemListArray,
        "PlayFabId": currentPlayerId	
    }
    
    try {
        server.GrantItemsToUser(grantItemsToUserRequest);
    } catch(e) {
        return 500;
    }
    
    // 6. subtract virtual currency
    const subtractUserVirtualCurrencyRequest = {
        Amount: serverPrice,
        PlayFabId: currentPlayerId,
        VirtualCurrency: "yourVirtualCurrency"
    }
    
    try {
        server.SubtractUserVirtualCurrency(subtractUserVirtualCurrencyRequest);
    } catch(e) {
        return 500;
    }
    
    return 200;
};