It couldn't - your blog isn't a Blogger blog, and the support article quite clearly states it's only for Blogger blogs.
Other CMS platforms have other means.
In any case, your feed does not even pass feedvalidator.
Since your Feedburner feed is already summary, couldn't you just provide Feedburner
with a summary feed to begin, instead of the full content of your (currently) 156 posts ?
Are using some plugin in NopCommerce to provide the feed ?
Each plugin would have their own settings to customize the feed,
so you'd have to ask them or NopCommerce forums for support.