Problem

Some Magento actions fail with "Access Denied".

Example:

Solution

This is caused by the API user not being given enough permissions.

1. Go to: System -> Web Services -> Users
2. Look at which "Role" the API user is assigned to.
3. Go to: System -> Web Services -> Roles then select the role from the previous step.
4. Make sure the Shopping Cart, Sales, Catalog Inventory, and everything in them is selected. Also, make sure Catalog > Product > Update is selected.
5. Click Save Role at the top after making changes.