[Airline] Southwest [Costs] The seat will be offered at the same price as the first one at time of booking it is not recommended to wait until you arrive at the airport; Southwest is known for being particularly inflexible on this issue. Contact for a refund of the cost of additional seating after travel, provided the flight is not entirely full or has not oversold. [Stated policy] 
Customers who encroach on any part of a neighboring seat should proactively book the needed number of seats prior to travel. (The armrest is considered to be the definitive boundary and measures 17 in width.)