Can I send an email to all my clients regarding the new Payroll fees starting next month through my QBO practice using the emails used to invoice my clients?

Hi Fred10, 

Thank you for reaching out on this. 

You can utilise the message session in your invoice template to include a message regarding the change. However, you'd still need to send the invoice one-by-one. 

Here is an article on invoice template for your reference:

Email and customise sales form

Hope this helps, and please let us know if you require additional assistance.