Shopify is a very popular site building platform for people interested in building eCommerce stores. It has been around for the past few years and seen significant growth in its user base over this time. You can earn a staggering 200% per sale for every new customer you refer to them, which means that there is up to $2400 per new customer on offer.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.