Paypal Plus Integration in the flutter framework app

I am using Fluxstore Prestashop - Flutter E-commerce Full App to make an app for Prestashop-based website. As a payment gateway, the app only has a function for cash. I want to add Paypal plus as a payment gateway. I asked the original development company but they are busy.
So, I am looking for someone who could help me Paypal plus function in the app.

Here is a link for the app code.
https://codecanyon.net/item/fluxstore-prestashop-flutter-ecommerce-full-app/27342379
I will give all code and access.