How can I send a notification to all customers

Hi lisler, thanks for posting on the Community

It isn't possible to send email notifications from QuickBooks Online, you would need to do this via a third party or your email provider. You can check out apps that integrate with QuickBooks over on our app store (please note, as apps are third-party, they have separate support and pricing). 

Thanks for reaching Intuit support. I'm just a post away if you have further questions or require additional assistance.