The server is prepaid, with no automatic renewal: it runs until its expiry date, shown at all times in the banner, and the button Extend adds the duration of your choice. No one takes anything from you behind your back, and in return, the expiry date is up to you.
Duration, discount, payment
The longer the duration, the higher the discount, up to the best one on the longest commitment. Bank cards and PayPal are accepted, and the summary shows the exact price before any payment.
Creepers, the in-house currency
Creepers are earned on the site, for example by voting for your server, and can be bought in packs. They pay for all or part of an extension: when you pay, the panel offers to deduct them from your balance. Your balance is shown in the account menu, next to your servers.
A server that has reached its expiry date is not deleted right away: a grace period keeps it available, long enough to extend it. Even so, it is better not to play with that margin, the banner warns you well in advance.