From Shopify Wiki

Setup

Please note that this does not work for Google Checkout based checkouts.

In order to offer FedEx-calculated shipping rates in your Shopify store, you will need to sign up for FedEx Web Services:

1. Register for an online account with FedEx, if you don't already have one.
   1. Go to fedex.com and choose your locale.
   2. Click the "register" link in the sidebar.
   3. Choose either "Open a FedEx account" if you're new to FedEx or "Use my account online" if you already ship with them. Do not select "Create a user ID only." If you do, you will not end up with the account number that you need.
   4. Follow the instructions to create your account.
      • Make sure you opt to create an account number during this process, and record it for later reference.
      • When it comes time to enter your credit card information, take note of exactly how FedEx has saved your billing address; you will need to enter it the same way later on.
2. Make sure you are logged in to fedex.com.
3. Follow this link and fill out the first form as shown in the screenshot:
4. Click "continue," accept the license agreement, then fill out the contact info form.
   • Enter the account number that you received when registering earlier.
   • Enter your billing address exactly as it was saved when you entered your credit card information.
5. When you reach the confirmation screen, you need to save or print out the provided credentials, in particular the Authentication Key. You will not have access to it later unless you go through this process again.
6. Check your email. FedEx should have sent you one called something like "Your developer Production Key Registration is complete." This email contains the rest of the credentials that you need.
7. Log in to your shop, click on preferences in the upper right, then click on shipping in the sidebar.
8. Click to activate FedEx-calculated shipping rates, then copy-and-paste the credentials you got from FedEx into the appropriate fields and hit save.
   • At this point you should get a message at the bottom of your screen confirming that FedEx has been set up.
   • If you get an error about invalid credentials, please make sure that you have put each credential in its correct place.

To change the rates that you offer your customers, scroll down to your list of regions at the bottom of the shipping preferences screen and click the edit link for a region's FedEx entry.