One $19 product is a coincidence. Five $19 products that share an audience is a business.
Audience first, catalog second
Pick a buyer you can describe in one sentence and stay there for a year. Every product you ship should be obvious to that buyer within ten seconds — otherwise you're starting over with each launch.
The compounding move
Product two costs less to launch because product one already paid for the audience. By product five, the cost of the next launch is mostly attention, not acquisition. That's the actual unlock.
When to stop stacking
When two products start eating each other's sales without growing the audience, the catalog is full. Time to either raise prices, bundle, or pick a new buyer.