Services renew on a cycle — monthly, annually, or whatever cycle you chose at checkout. Renewal is invoice-based rather than an automatic charge you cannot see coming.
A renewal invoice is raised in advance of the due date and emailed to you with a link to pay. Hosting is invoiced a short lead time ahead; domains are invoiced further ahead, because a domain renewal has a hard registry deadline and a longer lead time gives you room to act.
You can pay any invoice from the Invoices page at any point after it is raised. You do not need to wait for the reminder.
If you want to change cycle — monthly to annual, say — open a ticket before the next invoice is raised, and we will re-cycle the service so the change is reflected on it.
Billing