Ahh, dooh.
It seems like the source that the script rip data from by standart only contains 250 chars
If you eg. go to
https://www.amazon.com/gp/aw/d/0323353177/
And view it as user agent:
Mozilla/5.0 (iPhone; CPU iPhone OS 5_0_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A405 Safari/7534.48.3
The div with productDescription only contains 250 chars.
If you press the field you go to a new page with the path
https://www.amazon.com/gp/aw/d/0323353177/#productDescription_secondary_view_div_1520843164890
Here you have the full description inside the div productDescription_fullView
It should have a xpath
//*[@id="productDescription_fullView"]
A dirty solution in: https://github.com/discourse/onebox/blob/master/lib/onebox/engine/amazon_onebox.rb
Find the empty line 85 add
description = raw.at_css('#productDescription_fullView')&.inner_text
Change line 104
description: raw.at("#productDescription")&.inner_text,
to
description: description,
To have something like that
