I am in the United States, and after doing some research it appears that QBO is suppressing the reality of this situation.  Plain and simple, this is a Quickbooks problem.  Not a problem of their users like me that are trying to send invoices to customers, and it's not an issue on our customer's end. 

I've found a lot of communication surrounding this issue, and it's an issue on Quickbooks hands.  I use QBO myself, and when I use the ability to send an invoice to a customer from QBO I expect it to work.  I have myself copied on the message, my business partner, and my office manager.  Sometimes I don't receive a copy of the email, sometimes my business partner doesn't, sometimes my office manager doesn't, and after talking with customers, they are are also not receiving invoices.

A notice should be sent to all QBO users that this issue exists and a timeline provided of when this will be resolved.  Invoices to customers = accounts receivables that need to be paid as cash flow is paramount in any business.  Cash is king, and QBO support & engineering need to realize that and get with it.  If anything needs to prioritized to resolve it would be this issue.