After upgrading Checkout system from simple php application to drupal 8, we had our product informations into another drupal 7 based system.
This toolbox system in drupal 7 offer marketing Landing pages by content editors. Setting their headers, footers and product combinations as it was magazine site.
After Product selection we have to post this product information to drupal 8 checkout system, so on form_submit we redirect to a gateway php which translate everything into an searlized object encode and post to drupal 8 checkout system.
And atlast drupal 8 create product into its cart and sessions.