Adding price field on Drupal Commerce that affect Order Price
There is a contrib module developed espacially for shipping - Commerce Flat Rate


  This module allows you to define any number of flat rate shipping
  services via the user interface in conjunction with Commerce Shipping
  2.x. When you define a new flat rate service, you specify the base rate for it in the add / edit form. You can further modify the base
  rate (enabling things like weight or quantity based shipping) using
  rate calculation rules.
  
  This is functionally equivalent to using product pricing rules to add
  discounts or fees to base product prices before adding them to the
  cart.


There are few old but still actual video casts from rfay about Commerce Shipping. Watch it for a basic understanding:


Introduction to Commerce Shipping
Free Shipping Based on Order
Value or User Role
Shipping Discount for Item in Cart using
Calculation Rules