You can only import your transactions via Excel file. I'm here to help you accomplish this.
First, let's make sure to save your transactions via excel file. From there, here's how you can import them:
Once completed, you can now successfully upload your client's invoices and sales receipt transactions.
Here's an article you can read to learn more about how you can import your transactions: Import/Export Overview.
Let me know if you have any other questions. It'll be always my pleasure to help.