Hello @usernicky,
As of the moment, there isn't an integrated way to let the invoice automatically populate the rate base on the selected clients.
Even if your client has their rate, the invoice will show its amount base on the selected product or service. If you still wish to accomplish, you may want to create product or service for each of your client so you can assign and automatically populate their rate.
In the same manner, here's an article you can read to learn more about creating an invoice: Create invoices in QuickBooks Online.
On top of that, I've also included this reference for a compilation of articles you can use while working with us: Sales and Customer for QuickBooks Online.
If you have any other questions, please let me know by leaving any comments below. I'll be here to lend a hand.
