How can I get the certificate or summary page added to a completed envelope email?

There is an Admin setting that can be enabled, here are the steps.

1) Go to the Settings Tab, Signing Settings.

2) Locate and enable the setting under Envelope Delivery called "Attach certificate of completion to envelope"

3) Save

Remember this is an Account level setting so it impacts all envelopes on your Account and needs to be enabled by a DocuSign Admin on your Account.