At Amazon you can register as many so called tracking-id's as you want.
These codes act as your aff.link.
You can insert them on your pages as links, tie them to your own banners, whatever you want.
If you choose any of the promo-materials at Amazon-affiliates the selected ID is inserted in the code automatically.
You can always check if you inserted you aff.id correctly by clicking on the link that takes you to Amazon.
You aff.id shows up in the URL if all is correct.