Do you need to migrate data from Freshbooks to QB Online completely, OR sync data between Freshbooks and QB Online? You will need a different solution for each of them.

Just migrate old data. In fact the only thing I really want to migrate is old invoices and estimates. Thanks for the response.

For your reference

https://quickbooks.intuit.com/learn-support/en-ca/manage-lists/common-questions-about-importing-data...

You can import invoices

https://quickbooks.intuit.com/learn-support/en-ca/import-or-export-data-files/import-multiple-invoic...

but you need a 3rd party tool to import Estimates

https://transactionpro.grsm.io/qbo

Otherwise, consider purchasing a 3rd party service to migrate your Invoices and Estimates from Freshbooks to QBO.