Also note that you cannot post affiliate links on a free hosted WordPress.com. To do so is a violations of the Terms of Service which can result in your blog being suspended. In a nutshell no blogger initiated advertising, retailing or reselling the work created or services provided by anyone other than yourself is allowed on free hosted wordpress.COM blogs. E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by wordpress.COM
The advertising exceptions noted in support documentation are for high traffic blogs that qualify for and are accepted into the Ad Control program, and for extremely high traffic blogs that qualify and are accepted into the paid VIP hosting program.
If you do require an ecommerce site, advertising and/or affiliate links you can hire a web host and get a free software install from http://wordpress.org Refer to > http://support.wordpress.com/com-vs-org/