Regular plans are never ending and are very flexible (you can easily change the frequency and size). These plans are great for people who travel a lot.
Prepaid plans are for 3 months, 6 months or 1 year. They are cheaper and you pay everything up front so it is better for budgeting. Prepaid plans can be canceled and refunded (pro-rated for # of deliveries shipped) within the first 30 days from purchase. After that, there's no refund.
These plans automatically renew at the end of your plan.