Integrate POLi rest api in Wordpress

I need to integrate POLi internet banking gateway on my Wordpress website using REST JSon API.

Here's the link to POLi documentation:
https://www.polipayments.com/Developer

Please note that I am not using any e-commerce plugin and have no intentions to use a shopping cart plugin.
I want to use POLi for my custom checkout and will be calling it as and when need arises on a button click event.