Why use a landing page to build a list when you can just stick a button to take the user to the paymant page? Is the only reason so you can send other offers in the e-mails? Ain't it better to just have a button gong to the purchase of the product?
This is the only reason you can send other offers in the e-mails. If someone has purchased from you once, you can make him to order another product EASIER