Will entering 1 monthly lump amount in sales be workable in QB , because we have a separate sales system that already prepares the daily invoices.

It depends on how you will record it in QBO. You may create a montly sales receipt at the end of each month. Otherwise, you can migrate your invoice transaction data into QBO using an importer tool.

https://transactionpro.grsm.io/qbo

https://partners.saasant.com/affiliate/idevaffiliate.php?id=5051_2

Just my 2 cents. 

Hi simeontan,

You can record the monthly lump amount using the sales receipt if your customer pays immediately for the product or service or invoice if you need to bill your customer after the sale is made. 

Here's how:

1. Click the + New button.
2. Select Sales Receipt.
3. Choose the name of the customer or create a new one. If the customer is currently not set up in QuickBooks Online, select Add a new customer.
4. Enter all sales information needed to complete the sales receipt. 
5. On the Description column, enter the information for the monthly lump amount,
6. Click Save and close.

You can do the same thing once you want to enter an invoice: Create Invoices in QuickBooks Online.

Although sales receipts and invoices are used to track your daily sales, you can still follow the steps above to track it on a monthly lump amount.

You might want to run the Sales by Product/Services Detail report if you want to view the total sales: Run Reports.

Please touch base with me here for all of your QuickBooks needs, I'm always happy to help.