I am building a custom Shopify app to set custom shipping rates on Shopify.
I am using PHP to build app.
The app get zip codes from customers when they make checkout, and then calculate shipping rate from external API provider based on customer zip code and store zip code.
This API return shipping cost based 2 zip codes (pickup zip code and delivery zip code)
When customer make checkout, i am getting this custom shipping cost but sucked to set this shipping cost to shopify shipping rate.
I think it can be realized by carrier service api but i am not sure how to do it.
Who can direct me to do this?