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The categories are quite broad search terms. With a low domain and page authority you'll struggle to rank for those competitive keywords. The bumbf and links at the bottom of your category pages may be an issue.

You could essentially add them as Product Tags on the Product Level page only. If you don't hyperlink those keywords to anywhere, you should be fine. However, it would be even better if you could somehow incorporate these phrases as part of the product description. That would be the best option in my opinion.

Need to know which is best out of the two. I also read somewhere that JSON is becoming the standard. Confused!! Please help..

Just a quick tip. I wouldn't blindly post product links all over your social media platforms just for the sake of 'trying to affect your ranking' as more than likely it will come off quite spammy. I would probably focus on more creative strategies of getting people to either share or +1 your posts and pages through unique content, competitions etc. I imagine in the long run it would have much more positive implications for your main page's ranking.