When a user finishes their free trial of your product, they should be prompted to switch to a paid subscription. This is a huge opportunity to onboard paying customers and the completion of the free trial should trigger an automated email. In the email, make it easy for users to start paying. Make the experience of upgrading to a paid subscription seamless. Send them directly to a page where they can input their credit card information and continue enjoying your product. (Tip: this is also a valuable time to collect feedback.)