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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.
Just my 2 cents.
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.
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.
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