Twitter automatically truncates all links to a shortened link. If you are posting your affiliate links on Twitter, you may notice that they will end up giving you a 404 page.
One way around this is to use a link shortener first, like bit.ly. Twitter will not shorten an already shortened link and bit.ly links seem to work. You can customize the end of the link for free, or you can get a paid account and customize the front end of the URL.