The WordPress platform is the easiest method of setting up your website pages including high sales letters. So why not just use WordPress to setup all of your sales letters.
WordPress is suitable for website pages of all sorts including sales letters, landing pages, squeeze pages, confirmation pages, download pages, membership site pages, etc.
In fact with WordPress it is most often even much easier to create all of these website pages, as a webmaster can use many of the great WordPress plugins so as to automate all of these internet marketing chores.