The original question had to be edited. I think that Atwood includes the reader at the end of the work to involve us in the critical thinking process that satirizes the typically "Romantic" notion of "happily ever after." Atwood's presentation of this satire reflects how there cannot be a notion of "happily ever after." This is an impossible standard which is designed to create difficulty for the individual who cannot achieve it.
Through the incorporation of the reader into the final ending of the narrative, Atwood is asking the reader to delve into their own critical thinking skills. Such an analysis reveals that there is no such thing as a "happy ending." Rather, it is an external and unrealistic standard that is forced upon the individual. Atwood's incorporation of the reader is one in which they must understand how the basic condition of "happy ending" is fraught with falsehoods. In the end of the narrative, the inclusion of the reader helps to accomplish this idea. The reader becomes a part of the story for they are asked to critically dissect the arbitrary nature of "happy endings."