Shopify has introduced an API for Unity that lets developers sell merchandise for their apps right in their app. Developers that implement the Unity Buy SDK from Shopify can easily sell merchandise such as shirts, mugs, and toys without leaving the app. One example is Alto's Adventure [$4.99], which can sell merch such as plush llamas straight in the app now. Oh, and it does support Apple Pay so you barely have to do anything to get your sweet, sweet, merch on.
The elephant in the room ? which I'm sure Shopify will sell you ? is that there's something kind of funny about mobile games that cost a few bucks at most, if at all, getting an API to sell physical goods that would cost more than a few bucks. But people's value propositions totally change with having a physical, tangible product. And merch can be like consumable IAP for paid games, in a way. Even just implementing something that lets devs sell t-shirts easily like Amazon does (speaking of which, you should buy a TouchArcade shirt or the fabled Maglor INSTABUY shirt) would be a way for developers to help increase their chances of surviving and thriving in a cutthroat market.
Update: Article updated to clarify that Unity has no official relationship with this API from Shopify.