Welcome to the Community. I'm here to help with inputting your old invoices into QuickBooks Online.
One option that you have is to manually enter the invoices that you sent prior to having QuickBooks Online. Also, there are also third-party applications that can assist with importing invoices, which you can browse at the following link: QuickBooks Apps.
Since invoices can impact multiple areas, such as reporting and sales tax, you may wish to consult with your accountant to ensure the best path for your business.
I'm including some links that I believe may be helpful for you: