Build the first screen assuming it's all most people see, let the page test itself, save the card at first purchase, add a server-owned order bump, and deliver by token off the payment webhook.
What does somebody need to believe in order to buy this? Write the answer down. Every section of the page then has one job, which is to move a reader closer to believing it. Anything that doesn't do that comes off, however much you like it.
This part stands alone: if all you bought was this one, you'd have everything you need to build a page that sells and a checkout that doesn't leak money. It just won't tell you what to put in the proof boxes if you don't already have a worked ratio or receipt from elsewhere in the book.
What does somebody need to believe in order to buy this? Write the answer down. Every section of the page then has one job: move a reader closer to believing it.
You don't need to build any of this yourself, but you need to know which column each piece sits in, because the middle column is where money goes missing quietly.
I built analytics models for banks and miners before this. Every step in The Book Is the Funnel is one we ran on our own funnel first, failures included. This part is sold on its own because most people need one piece of it, not all sixty at once.