Create multiple sales orders of same customer and you can issue a single merged invoice.
This is a kind of normal practice that most of the regular customer never needs invoices on daily basis, But they are expecting invoices on specific time interval like weekly once or monthly one etc.
Here is a list of sales orders of same customer to be invoiced.
Now you can choose the sales orders whose invoice is not generated when you create invoice. When you choose the sales order, The invoice line auto fills all the data and you can create invoice seamlessly.
No more mess up, single merged invoice. Many sales orders in single invoice. Customer will be more happy that they can maintain one invoice of one vendor for each month or week etc.
So this will saves our much time as well.