Amazon is expected to dominate — grabbing as much as half of all online sales by the end of the year, according to consulting firm Bain & Company.

But that doesn't mean others aren't putting up a fight. Target, for example, is temporarily offering free two-day shipping for the holidays by dropping its usual $35 minimum purchase requirement. Amazon responded by dropping its $25 minimum, but Walmart is still sticking with its $35 requirement.