You’ll only be billed for any usage of the service after 12 months.
If you’re just signing up to get an Access Key ID then you won’t be billed at all because it’s free.
If you use the service to store files etc then you will start getting billed on a monthly basis after your first year.
I think it’s only silly amounts, like for storage it’s only $0.09 for each gig you use every month.
For full pricing go check it out here
Hope this helps 🙂