WP connect payment system

There is a plug-in personal account WP Recall to it has instructions on connecting payment systems:
https://codeseller.ru/post-group/porya...steme-php/
There is a payment service paykassa.pro to it there is an API:
https://paykassa.pro/ru/developers/
The challenge is to integrate according to the instructions.
Offer price and timing (need as quickly as possible)