Gmail Customer Lookup is a Chrome extension that shows Easy Digital Downloads (EDD) purchase history next to incoming emails.
To configure, you need to obtain an EDD Key and Token by visiting your own user profile page in your EDD WordPress site. In your profile page (Users -> Your Profile), find the Easy Digital Downloads API Keys section. Check the Generate API Key checkbox, then click the Update Profile button.
You should now see three different codes in the Easy Digital Downloads API Keys section. Ignore Secret key, but copy the other two values in to the boxes below - Public key and Token.
You will also need to enter the EDD API endpoint URL below. This is normally your WordPress website URL with /edd-api/ appended. The default below may already be correct.