Amazon affiliate program
Amazon affiliate UK is exactly the same as any store related to locality, so you will have to sign up to your local amazon in order to start earning. To my understanding the organization doesn't have a central payment and sale system for the purpose of legal requirements such as tax.
The commission can vary depending on the number of refers made, the product and possibly other factors I am not aware of, It starts from 5% and it can go up to 15%.
Once you have signed in, you can start generating links for products and categories. Remember you can have more than one Associate ID for your links. The usefulness of having multi IDs is that you can segments your projects and websites by creating a key for each project. Tracking ID is your associate ID and this might be the source of confusion because it has more than one term for the same thing. Also don't confuse associate id with your log in id because your log in ID is your email address and it is unrelated to the product api and its keys. If you are going to deal with api directly then you will need Amazon AWS Access Key ID and Amazon AWS Secret Access Key. To obtain the other two keys a trip to Amazon web services and registration is required on top of your registration to associate program.
Again in order to use Drupal amazon store, a registration is required in both the associate program and the web service division. Web services is needed for security access to the apis. Make sure you keep those keys secret and never disclose them to anyone outside the trust circle.