payment flow on single page

One time payment
Customer enters card details and presses "pay one time" button
1/ [server-side] create a PaymentIntent
2/ [client-side] call confirmCardPayment() and pass in the cardElement
Done

Subscription
Customer enters card details and presses "subscribe" button
1/ [client-side] create a PaymentMethod token
2/ [server-side] attach that PM token to Customer
3/ [server-side] create a Subscription on that Customer and card
4/ [client-side] case of first payment requiring authentication, if the subscription.latest_invoice.payment_intent is in "status: requires_action", then back on your webpage, call confirmCardPayment