If you are trying to set up an Amazon Store that's a no go on a free hosted WordPress.com blog. We cannot post any iframes on free hosted WordPress.com blogs. The software will strip the code out to preserve security as this is a multiuser blogging platform.
The only exceptions to the affiliate links policy > http://en.support.wordpress.com/affiliate-links/ are found here in italics under blogs that are not allowed:
Affiliate marketing blogs: Blogs with the primary purpose of driving traffic to affiliate programs and get-rich-quick schemes (“Make six figures from home!!”, “20 easy steps to top profits!!”, etc). This includes multi-level marketing (MLM) blogs and pyramid schemes. To be clear, people writing their own original book, movie or game reviews and linking them to Amazon, or people linking to their own products on Etsy do NOT fall into this category. Here is a thread in the support forums that talks more about which affiliate links are OK or not OK. http://en.wordpress.com/types-of-blogs/
The only things you can sell on WP.com blogs are products that you, yourself create and/or services that you personally provide. For ecommerce, you will need an independently hosted blog using WordPress.ORG software.
For your selling your own creations see http://en.support.wordpress.com/paypal/
If and only if you are marketing only what you create and have a online shop with a shopping cart somewhere else online you can create a custom menu and add a custom link to it.