The economics of 1 time purchases for small pieces of content (aka micropayments) don’t work - many have tried - nobody has ever succeeded, which is the main reason we focus on subscriptions. This is covered in more detail on Ghost: The Creator Economy Platform
The economics do work for larger pieces of content, like albums or movies, but at that point you’re talking about ecommerce - which is not what Ghost is for, and you’d be better off using something like Shopify