We've added an optional email renewal reminder to the VPS management area.
Using it is very simple. Just enter any email address and click 'Enable email notifications' to be reminded before your server is due for termination.
-- BitVPS
How many days before expiration it is possible to pay and continue using server?
You can pay as many days in advance as you like. You can purchase a server today for X days and renew it for another X days right away.