The n.1 marketing mistake is the intention to casually monetize on what the customer buys, without having a front-end and back-end strategy.
The front-end is the product we should propose to prospects as magnet, in order to attract them.
It like a trojan horse or a mouse trap that attracts the customer in your world.
With this type of offer the goal is not to make a proifit, but to break-even (just not lose money). In some cases you can even make a loss, because you have a solid plan to sell him/her some other stuff.
Each real marketer has three types of products: one to advertise, one to sell and one to make profits.
Cinemas advertise movies, but make money on popcorn and soft drinks
Car dealers advertise cars but make money on upgrades and maintenance
Fast-food chains advertise hamburgers, sell fries with hamburgers, but make money with soft drinks
Post here if you want to discuss your business funnel structure