Real-time shipping quotes at your store checkout can give you the competitive edge, or at least stop your profit bleeding out through not charging enough for shipping.
The Smart Send Shipping Unlimited app can do this for you - but you first have to make sure that Shopify have activated 'Carrier Services API' on your store account.
Carrier Services gives your store the ability to request accurate shipping prices from the Smart Send app, given the items in the cart and the destination. You can choose to have the app return the actual rates provided, add a handling fee, provide a discount or return a flat rate.
To activate Carrier Services, simply contact Shopify support. It's free if you have a Shopify Plus account or are on an annual subscription.
See also the Shopify page: