UPDATE: Lasso doesn't solely rely on the Amazon API anymore. If you have no more credits, Lasso will still grab the price, image, and title and be compliant with the Amazon Associates terms of service.
Even if you didn't want to connect Lasso to Amazon Associates, you can always add Amazon products and still retrieve the latest price and image.
First, click the "Add New URL" button at the top right corner on any page in Lasso.
Simply add any Amazon Associates URL (the full URL, not the shortened one) by adding a new URL.
Then you'll see the Link Details page with pre-filled information straight from Amazon.
If you don't display the proper image and price, you'll be going against Amazon's terms of service. This way you still get updated information that's compliant with Amazon's Terms of Service - you just don't need your own API keys for it.
NOTE: You can still use the shortened link or long link from SiteStripe, but you'll be making more work for yourself. ;)