How can I send email notification to all my customer at once

It's nice seeing you posting your concern in the QuickBooks Community, @MaximillianT. 

Let me assist you with your concern about sending email notifications to your customers. 

I know a way how you can send mass emails in QuickBooks Online, and I'd be glad to guide you on how: 

1. Go to Sales and select the Invoices tab.
2. Select the customers to whom you want to send the emails.
3. Click Batch Actions. 
4. Tick Send Reminder, and you'll be asked to choose an email service.
5. Once done, click Send. 
   
   

If you want to add a Carbon copy (Cc) and Blind copy (Bcc), enter the addresses in the CC and BCC field. 

For more details about doing batch emails or printing your invoices and other sales forms to make your task easy and efficient. I've got this handy resource as your guide: Email or print multiple sales forms in QuickBooks Online. 

Moreover, I have an article that will help you account for your customer's payments: Receive invoice payment.

Click the Reply button if you need more help managing your invoices or have any QuickBooks-related concerns. Take care and have a pleasant day.