I can see the urgency of getting this resolved, @erinwal and @HydSol.

I checked the status of the investigation about unable to send invoices in QuickBooks Online (QBO), and it's still in progress. Rest assured our engineers are doing their best to get you back on track as soon as possible. In the meantime, you can download the invoices as a PDF and attach it to an email outside of QuickBooks Online. Please know that if you opt for this workaround, you'll need to manually apply the payment to the invoice once you receive it from your customer. 

To receive updates once the issue is resolve, you can subscribe to this link: https://status.quickbooks.intuit.com/.

For future reference, check out this article to learn how to receive invoice payments in QuickBooks Online. 

Thank you for your patience and understanding while we look into this further. If you have any further concerns, feel free to reach back out. Have a great day.