Revision of the basket, payment methods Opencart 2.1.0.2.1

The essence of the problem: When you select Simple in the basket a certain number of months in the methods of payment: "payment in part" to increase the price of goods at a specified percentage

Website: https://rezina-ua.com

TK: https://docs.google.com/document/d/1JGQ9p3mKdnkXk5iycp1bHh1lt0qIj-5tj9moZRlXEWQ/edit?usp=sharing