Promotional links are NOT allowed in articles
The executive summary of this post is that links are not allowed in articles in almost all situations. You must only add links to the "Author Byline" section.
The only time it is ok to add links in the main body of your article, is if the links add value to the reader. An example would be if you are comparing products and wanted to give your readers a more in depth look at specs. Instead of copying all of the specs for each product into the article, you could link to the specs page for each.
If a link is added simply to promote, sell, or boost search rankings for a product or service, the article will be declined. For more info see the article submission guidelines.