I run two eCommerce sites with one-off payments for products.

I run three content sites that run ads for revenue.

I run one web application where you deposit money into a pool and service usage deducts from that pool.

I run one web application which is free with optional paid subscription upgrades.

I run one web application which requires a paid subscription.

I never decided to add on a revenue stream, I decided to build products, and chose the business model that made sense for the product and customers that would be using it.

You should be able to figure out what the sites are by poking around my profile. Feel free to ping me if you had some specific question.