Hi,
The URL unit cannot find products from JavaScript generated contents and the page you posted (https://www.amazon.es/gp/goldbox) renders outputs with JavaScript. So the plugin does not see products in that page. This Chrome extension (https://chrome.google.com/webstore/detail/quick-javascript-switcher/ahjfodbngfpdppljbkhcfhcfdagfgcnj) is handy to toggle browser JavaScript so that you can easily check if shown products are displayed with JavaScript or not.
That particular goldbox page for US offers Email subscriptions. And there are plugins that allow you to create posts from Emails. You can have a hidden site that nobody accesses but only you and create posts of Amazon deals with Email. Then point the top page or suitable page with the URL unit.
Another solution would be to use a web proxy that returns JavaScript generated contents. If you are interested, I may be able to write such a tool. In that case, shoot me an email via the contact form so that we can talk about it.