void GetVcStore()
{
    var primaryCatalogName = "TestCatalog-001"; // In your game, this should just be a constant matching your primary catalog
    var storeId = "Potion Store"; // In your game, this should be a constant for a permanent store, or retrieved from titleData for a time-sensitive store
    var request = new GetStoreItemsRequest
    {
        CatalogVersion = primaryCatalogName,
        StoreId = storeId
    };
    PlayFabClientAPI.GetStoreItems(request, LogSuccess, LogFailure);
}

void DefinePurchase()
{
    var primaryCatalogName = "TestCatalog-001"; // In your game, this should just be a constant matching your primary catalog.
    var storeId = "Potion Store"; // At this point in the process, it's just maintaining the same storeId used above.
    var request = new StartPurchaseRequest
    {
        CatalogVersion = primaryCatalogName,
        StoreId = storeId,
        Items = new List<ItemPurchaseRequest> {
             // The presence of these lines are based on the results from GetStoreItems, and user selection - Yours will be more generic.
            new ItemPurchaseRequest { ItemId = "Small Health Potion", Quantity = 20,},
            new ItemPurchaseRequest { ItemId = "Medium Health Potion", Quantity = 100,},
            new ItemPurchaseRequest { ItemId = "Large Health Potion", Quantity = 2,},
        }
    };
    PlayFabClientAPI.StartPurchase(request, result => { Debug.Log("Purchase started: " + result.OrderId); }, LogFailure);
}

void DefinePaymentCurrency(string orderId, string currencyKey, string providerName)
{
    var request =new PayForPurchaseRequest {
        OrderId = orderId, // orderId comes from StartPurchase above.
        Currency = currencyKey, // User defines which currency they wish to use to pay for this purchase (all items must have a defined/non-zero cost in this currency).
        ProviderName = providerName // providerName comes from the PaymentOptions in the result from StartPurchase above.
    };
    PlayFabClientAPI.PayForPurchase(request, LogSuccess, LogFailure);
}

// Unity/C#
void FinishPurchase(string orderId)
{
    var request = new ConfirmPurchaseRequest { OrderId = orderId };
    PlayFabClientAPI.ConfirmPurchase(request, LogSuccess, LogFailure);
}