You gotta either build monetization right into the game or offer an easy way to buy it. There's a reason why payment processors and stores in general try to reduce friction to a minimum.

If you wanna pay for something you either do it right away and without much hassle or very likely never. Personally I would even rather buy a game right away and be dissapointed (and potentially revisit it later) than download a demo first. I realize that this is a privileged position to be in, but either way it is an advantage for you if I don't think about spending money too much