What is the authors purpose in writing Skipping Christmas?

We can probably guess why the author wrote the story, based on his words during the premier of Christmas With The Kranks.
The movie Christmas With The Kranks is adapted from Grisham's story Skipping Christmas. During his interview at the movie premier, John Grisham lamented the commercialization of Christmas. Although he admits that skipping Christmas entirely isn't the answer, he expresses his belief that there should be a better way to celebrate the holiday.
The commercialization of Christmas often leads to unnecessary stress for many families during the holidays, especially in financial terms. Essentially, the reason for the holiday has been lost, and Grisham points this out in his story. In Skipping Christmas, Luther Krank daydreams about the joy it would give him to avoid all the stresses commonly associated with Christmas:

No trees, no shopping, no meaningless gifts, no tipping, no clutter and wrappings, no traffic and crowds, no fruitcakes, no liquor and hams that no one needed, no "Rudolph" and "Frosty," no office party, no wasted money...

Luther reminds his wife, Nora, that they spent sixty-one hundred dollars on Christmas the year before and have "precious little to show for it." He also admits that he isn't too happy about the loss of time, "the traffic, stress, worry, bickering, ill-will, [and] sleep loss" associated with celebrating the holiday.
Luther's words echo Grisham's in his interview. In the story, certain materialistic traditions have become so entrenched in the social consciousness that the Kranks suffer great disapproval from members of their community for wanting to skip celebrating the holiday. So, it can be inferred that Grisham's humorous story is an attempt at highlighting the problems associated with commercializing Christmas.