I for one find it a huge relief not not have to negotiate the purchase price.

Costco's power in the auto industry is growing. 

Costco sold 465,000 vehicles through partnerships with auto dealers in 2015, a 16.8% increase from the previous year.

That's not far behind the No. 1 auto retailer in the US, AutoNation, which sold 533,000 vehicles in 2014. 

Costco has one major perk that traditional car dealerships lack: fixed prices. That means customers can skip the bargaining and upselling that is expected at traditional auto dealers. 

The company sells cars through 3,000 dealerships to Costco members across the US.

Costco also announced that members bought or leased 58,000 vehicles during its 2015 GM Holiday Sales Event, a 34% increase from the previous year's holiday-season promotion. More than half of the people who purchased vehicles through the event said they switched to a General Motors brand such as Chevrolet, Buick, or Cadillac because of Costco's promotion.