The "shop in shop" model is where a brand owner or retailer takes space in another retailer's store. This often takes form in an in-store display of some sort. There are benefits to both retailers e.g. shared costs, shared marketing and demand-generation, and speed to market.
Most artisans and artists use this method to sell their products.
For example, local jewelry makers might try to sell their collections by creating a display in a local coffee shop.
Deutsche Post is the world’s largest courier company. They utilize the model by displaying products from other brands in their locations.
This strategy has a long-standing history in commerce and it is utilized in a multitude of ways.
This strategy is best used when partnered up with complementary services or products.
For example, most postal services have displays of greeting cards that are usually owned by different brands (i.e. Hallmark).