Changing the paths for checkout
If you are wanting to change the paths that checkout uses, you may want to check out Commerce Checkout paths:
This module allow users to create a custom paths for Drupal commerce checkout pages(make it more "friendly" rather default "checkout/ID/PAGE_ID").
Adaptive checkout workflows and pane configuration
Out of the box, commerce give you a single workflow that applies to any checkout scenario. Commerce Checkout Multilane allows you to customize this based on the order information.
This module provides means to create additional independent checkout sequences (lanes) with different pane visibility and order. With this module it is possible to, e.g., provide a separate "Express" lane used for one-click checkouts or donations.
Selection of the lane is done through the checkout-lane entity property on the order. The property should be populated using an appropriate reaction rule, e.g., before a new order is saved or when a product is added to the cart.