Refer to the documentation on the Shopping Cart Plugin site

This plugin adds PayPal Express Checkout to your Shopping Cart.

To install this plugin, just download the zip version of this repository and unzip it under /your/site/grav/user/plugins. Then, rename the folder to shoppingcart-paypal.

You should now have all the plugin files under

/your/site/grav/user/plugins/shoppingcart-paypal

In the Admin, go to Plugins and configure the plugin with the required information, then enable it.