Hi there, @tim19422.
I appreciate you for using QuickBooks for your business.
At the moment, the option to sync invoices from QuickBooks to QuickBooks is unavailable. Hence, you'll have to enter them manually into the system.
I can see how convenient it would be to just sync invoices when both parties are using the same platform. I'll share your idea to our engineers so they can look into it and see whether this can be possible in the future updates.
For future reference, check out these articles to learn more about managing your transactions in QuickBooks Online:
Get back to me if you have more questions about invoicing. I'll be there to help you again.