This is the perfect place to add it. Link to your sales page or directly to the offer via the call to action button. Choose your action to be landing page or checkout. Add a picture if you like (click on show image above)

Just delete or hide whatever you don't want. You can always add it back in later or customise the sidebar to show exactly what you'd like. You can even turn the sidebar off and on. There is a separate sidebar for the blog home page AND the individual blog posts. It's a great place to include calls to action to get people to join your list OR promote your offers. Remember you can use images that are clickable instead of text.