Google AMP technology was designed for the Static Pages only. Therefore, AMP Magento Extension works only on Homepage, Product List Pages, Product Pages and CMS pages.
The pages like Shopping Cart and Checkout are dynamic pages, and they cannot be adapted to work with AMP technology yet. Once Google adds the support of the dynamic content – we will certainly provide our customers with the update of our AMP Magento Extension. For now all the dynamic pages will be served as non-AMP pages.