Are you able to expound more on what you are asking regarding promoting free shipping?
If it's about the verbiage you are after, free shipping, is what I would prefer. It clearly states that your shipping will be free in whatever context you put it in. Free plus shipping seems to indicate there is a free item plus shipping.
If I somehow misunderstood your query, apologies for the misunderstanding. I look forward to getting more info from you.
When it comes to advertising and shipping however, I have found these articles that you may find useful:
These articles go over some shipping strategies and tips as well as how best to display it on your page. Definitely give these a review for more tips regarding shipping and marketing.
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