You are using Email to Case Premium with Case Feed and are curious if the packaged Canned Comments feature can be accessed from within the Answer Customer publisher action.
The Canned Comments feature included with Email to Case Premium is specifically available for those utilizing the New Comment page (on a non-Case Feed based layout). Canned Comments are not available for those utilizing the Answer Customer publisher action on a Case Feed based layout.
When utilizing publisher actions with Case Feed, Salesforce offers a feature called Quick Text that includes some similarities to Canned Comments.
Please see the following documentation on how to get started with using Quick Text.
Note: Quick Text is only available in Salesforce Classic at this time. Please upvote the following idea for support within the Lightning Experience.