Everything about your plan lives under Settings → Billing. There you can see your current plan and renewal date, your credit and API-call usage against what your plan includes, and your team-member seats, template count, and branding status.
Payment and invoices are handled by Stripe. Click Manage via Stripe to update your card or download invoices, and use the same page to cancel your subscription.
How charges work. Your plan's base subscription is billed annually. Any credits you use beyond your plan's included amount are billed monthly as overage.
Consumption controls decide what happens when you hit your plan's cap. With Auto-upgrade at cap on, reaching your tier's credit cap moves you to the next plan and applies its better rates retroactively, so you are never worse off for it. You can also set your own lower caps to avoid surprise overage.
For plan prices and what each plan includes, see the pricing page.