Private app need to get customer shipping and item at checkout page

[zeeshandar]

I am developing a shopify private app and have a couple of questions.

The purpose of the app is to pull the delivery types (Next Day Air, 3rd Day Standard etc) and rates from our courier company. Rates are based on shipping zipcode and item weight.

Question:
At checkout page (where customer have to select the one of service types) how do I call the shipping and cart items in to my private app. So I can get the zipcode and item weight to calculate the rates.

Enabled scopes: unauthenticated_read_checkouts, unauthenticated_write_checkouts