Unable to exit from customer after making an invoice/estimate. Return arrow in top left is off screen

I'll help you manage your invoices and estimates, @ttroyz.

I suggest restoring down your browser window. This way, you'll be able to see the return arrow of your browser. Then maximize it to view it back in full screen. I've added a screenshot below for your visual reference. 

You can also click the refresh icon from your browser to refresh the page and isolate this issue. 

Once you're all set, you might want to check out this article that can help you in seamlessly account for your customer payments: Record invoice payments in QuickBooks Online.

I'm always around to back you up whenever you need help managing your invoices and QBO. Have a dazzling day ahead!