Development of custom PayPal payment process plugin on a Wordpress site

I need to integrate a simple PayPal payment process on a Wordpress site. Here are some infos: - Wordpress already installed - Custom post type "product" already created - Custom post type "order" already created - Design not needed (I will style the rough code) - Items can be added to cart - Cart contents will be saved for around a week so when the user come back to site he will retrieve the products in the cart - Orders will be saved to give custom post type - Paypal chained payment will be setted up - No simple html code: means I dont want a simple form for the paypal checkout because the user could change the total amount via google inspect (as example) I dont want to use any plugin. All files must be included in the theme directory.