You must know that the contents within an iframe is loaded directly from third party website.
In your case, your store is loaded directly from Amazon.
The codes of your store are not written with responsive in mind, the widths of all elements are written in fixed values and not in percentage.
The codes within an iframe is loaded directly from third party website, it’s not physically on your website.
It will not respond to your browser view port size.
It’s not possible to make it responsive.