Import Customer Invoices : Sample Files

Before you start importing you need to make sure your file is in the correct format:

Save your list of invoices as a tab-separated text-file (.txt) with your information arranged in columns and no empty rows between the invoices.
Use UTF8 as encoding if you have any special characters.
If you are a Mac user please save your file as a Windows Formatted Text (.txt). 

Remember that there are many prerequisites for the import to work.

Read more here.