Amazon has introduced a startling new perk for members of its $100/year Amazon Prime service: a 20 percent discount on preordered and newly-released games. Physical copies of video games will be automatically discounted during their preorder period and for up to two weeks post release, according to the company's FAQ on the new promotion. 

The offer is only available for U.S. Prime members, but that's a significant number of households. Though the company does not disclose the number of Prime subscribers, its U.S. base is estimated to be between 40 and 50 million.

It is worth noting that the discount does not apply to hardware or digital downloads. 

Retail competitor GameStop managed to eke out gains during its holiday period, but Amazon's aggressive move seems likely to have a significant effect on game sales. 

Though you can sell back used games to Amazon, it's done via the mail; GameStop has long held that its advantage is its preowned ecosystem, which generates store credit which it says is largely used on new titles -- and which attracts consumers.