Because of an issue with WHMCS, the software we and many others use in the hosting industry, it seems a large number of customers have not received invoice notices. What we have observed is that the system generates the invoice automatically, as it should, but fails to complete the next step in automation which is to dispatch the invoice as an email to the customer. We are working with WHMCS now to fix this issue as it's having a great negative impact on our business and as well as for your user experience.
We've manually dispatched all due invoices in bulk just now, so some users may receive more than one notice for the same invoice. For others, this may be their first received notice.