If you have chosen a topic that addresses the needs or wants of a specific segment of the population and if you have set up your opt-in software correctly, you have a list of potential customers. If they have subscribed to your newsletter, then they obviously believe that your have information about a subject that is near and dear to their hearts. They must consider you to be an expert maybe a guru. If you have worked hard at building your customers belief in your own credibility, then you should be able to make a nice profit from your newsletter.
You can advertise products and services related to your topic on your website. This can be done with logos and banners that contain links to the products and services. When visitors to your site click on an advertisement, you get credit for a lead and if they buy the product or service you get a commission on the sale.
You can use your newsletter to recommend products and services to the members of your list. Some companies will happily pay you for simply recommending their product or service to your list and give you a commission on any sales that your recommendation generates for them.