Pay-as-you-go subscriptions 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 subscriptions are 6 months with a 7th month free. They are cheaper and you pay everything up front so it is better for budgeting. Prepaid subscriptions can be canceled and refunded (pro-rated for the number of deliveries shipped) within the first 30 days from purchase. After that, there's no refund.
Prepaid subscriptions automatically renew at the end of your term.