Thank you for joining the thread, @samhui.

As mentioned by my colleague @SophiaAnnL, you can import your invoices using the Intuit Interchange Format (.IIF) files. Be sure to check the mapping for a successful import. 

You may check the sample header that you can use in the process: IIF import kit, sample files, and headers.

To give you more details about importing .IIF files in QuickBooks Desktop, please refer to this article link: Improved IIF Import in QuickBooks 2019.

Let me know if you have additional questions. I'm always here to help. Have a nice day.