How to integrate delivery generated by api in opencart?

Question

PHP

Greetings. I get delivery options via api via ajax (the price depends on the city and weight).
I can not figure out the logic of opencart at this moment.
when confirming the order, the system issues that the method is not selected. What can you think of? or who knows how this process happens in opencart? I see it when choosing a method and writes to the hidden input delivery in cookies.