Build Credit Options for Custom Payment Gateway

I need to build a calculator with credit options within a Custom Payment Gateway in Woocommerce. The calculator must calculate the Sub-Total into credit quotes; which are calculated with many equations (not a simple one, I will provide those formulas in excel); so Users can see their subtotal into 4 options of credit (12, 24, 36 or 48 months) with its respectively monthly quote amount. This should appear in the Custom Payment Gateway I use (a plugin called Custom Payment Gateway Pro from WP Ruby).
The site is in production (live), so any test or change must be done very carefully. Any change in the console must be supervised live, so your screen will be shared while working on it.