Assuming you already have a product in the catalog, but want to either give them a special price or add a custom Download:
What this does is keeps the "special" one from showing up in a search or the browsing navigation, but if someone has a direct link to it (via your newsletter), they can still buy it.
- Copy the Product
- Rename the new (copied) product so you can distinguish it from the main one
- Add any copy to the bullet points or description to mention the additional download
- Go to the Inventory tab
- UN-check "Available for purchase"
- A new option will show up: "Still allow visitors to view and purchase if they have a direct link"
- Check that box
- Click the "Public View" link in the upper-right corner
- Copy the URL for that product and paste IT into your email newsletter