public Dictionary <int?, DatabaseModel.Vendor> GetActiveVendors(Dictionary <int?, Customer> allActiveCustomers, VendorService vendorService, RecurringInvoices.Frequency paymentFrequency)
        {
            var activeVendors = vendorService.GetByPaymentFrequency(paymentFrequency)
                                .Where(x => allActiveCustomers.ContainsKey(x.QuickBooksOnlineId))
                                .ToDictionary(x => x.QuickBooksOnlineId);

            return(activeVendors);
        }