I'm new to odoo and I'm not a good user of python

I'm using this ERP to manage my car rental business

So I want to add more conditions to the (Rent A product) Action

now it's limited to only how many days I rented the car, I want to add another condition to limit the customer with only 200KM per day, and make him pay for the extra distance.

I'm using the online custom plan

I want to know If I have the ability to edit the source code and from where?

(I will hire a programmer for editing but I wanna know if I can edit first)