Compute (Instance type) pricing is per service, per instance, per month, prorated to the second. This means we charge for the time each paid instance is provisioned at the end of each month, not in advance.
Current usage, upcoming (projected) monthly charges, and previous invoices are available to view at any time on your billing page. You can downgrade or suspend services anytime to prevent them from accruing more charges.
The only real overage consideration would be bandwidth. Bandwidth is separate from compute cost, and 100GB is included with an individual account each month. Most customers don’t use all their bandwidth each month, and as above, current bandwidth usage can be seen on your billing page. Going over the included 100GB in a calendar month will add another 100GB block at $30 USD. You will be emailed if you get close to your initial bandwidth limit.