I don't think that is even limited to titles that are in the Game Pass catalogue. The main Game Pass page says subscribers get 20% off Xbox One game purchases and 10% off add-ons. I think that applies to everything in the digital store. I was excited about this too because I currently use Amazon's 20% off new games for Prime members deal pretty often even though I strongly prefer to buy my games digitally. The fine print at the bottom of the Game Pass page says this doesn't apply to games within 30 days of release though. That is still a nice perk, especially if it stacks on top of other sales, but it is a limitation that doesn't apply to Amazon or Best Buy.