Billing & subscriptions

What payment methods are available? How does subscription renewal work? What happens if an auto-renewal payment fails? What happens if I cancel my subscription? How do I stop a subscription purchased through the App Store or Google Play? Can I increase the limit on a specific feature? Can I get an invoice for my payment? What happens when I upgrade an existing subscription? Why does the final price differ from the listed price? Can I get a refund? Can I use my subscription on multiple platforms?