I'm working for a client who uses the Shopify Storefront API. In that case we have our own frontend for the shop and use the API to handle the checkout session. We also have our own cart page from where the user is then directed to Shopify checkout.
As far as I know it is possible to show a Paypal Express Button in the Shopify cart page. Since we use an own page for the cart my question is: Is it possible to use the Paypal Express button outside of Shopify ?