There are three types of subscriptions available:

1) Annual plan: the price is  GBP 13.99 per box.

2) Six-monthly plan: the price is GBP 14.99 per box.

3) Monthly plan: the price is GBP 24.99 per box.

The  boxes are sent out every month regardless of the chosen plan. The 6-monthly and annual plans allow you to save money, but have a minimum duration of 6 and 12 months respectively. 

There is no commitment of permanence, although an early cancellation of the 6-monthly and annual plans have an additional cost.