Mattd ,
Oviously Wal-Mart doesn't --- if it's the pistol grip that is the most appealing to you and you don't want a regular shoulder stock , then plunk down the $130 as it is a fair price.
Many find the pistol grip only shotgun a problem to shoot and unpractical outside of Hollywood . The point being that most would either want to , or end up changing to a standard butt stock for a lot of their shooting. By the time one would purchase the stock seperately the cost would be up at a level where you could buy a new Mossy just about anywhere making the $130 original cost no bargin & perhaps even a bit high .
If the gun in question has a "stock with a pistol grip" - that's a different matter, and the price would be very good at $130.