Hello Mike,
The user who sent the invoice and the main admin user in Method get copies of the invoice emailed out. The administrator of the Method account is the User with the (admin) beside their name in the Users menu. I believe you can turn it off with some customization. Usually people create filters in their email to collect them in so it doesn't clutter the Inbox.
Here is the action from the Save, Email & Close button on the invoice screen that sends it to the logged in user. Note action 29 and 30.
I hope this helps,
Greg