Where are you looking for this invoice? When logged in as account owner or account manager, you can email yourself a copy of an invoice through the "View Billing Activity/Statements" link under your My Account tab.
You will then select the time period you are interested in with the drop-down menu or the calendars and click Show.
Doing this will then open to your invoice and a place where you can enter an email address of your choosing in the field right about your invoice. Enter in your email address then click Email and choose what you'd like to email: All Activity, Payments, Invoices, or Invoices with Detail.
Were you able to email yourself your invoice after following these steps?