Can I pay annually for the Full Service?

Not at this time.  Some of the amenities that come with the Full Service subscription are paid for on a monthly basis.

There are plans in the future to get some of those amenities in bulk which could then be used for an annual subscription.

Until then, you will have to have a monthly subscription.  If you would rather not pay monthly, then you can try the Self Service option which is only a onetime fee.