As an extra and For good measure

For good measure and as an extra are semantically related In some cases you can replace phrase "For good measure" with "As an extra", this phrases are similar.

For good measure adverb - In addition (as to close a deal).

As an extra and for good measure are semantically related in in addition topic. Sometimes you can replace phrase "As an extra" with "For good measure", this phrases are similar, if it concerns topics such as as a bonus.