What the session collects from the customer.

• PAYMENT: Collect a one-time payment. The session manages a payment_intent.
• SETUP: Collect and save a payment method for future use without charging. The session
  manages a setup_intent; no amount is charged.

Client supplied identifier to cross-reference the checkout to internal systems (like an order id for
example).

Amount to be captured in minor unit of the currency (ex: euro-cents for euros). Required in PAYMENT mode;
must be omitted in SETUP mode.

Example: 300 (as in 300 pence for £3.00)

ISO 4217 three letter currency code. Must be supported by at least one payment rail associated with
the account. Required in PAYMENT mode. Optional in SETUP mode: when set, restricts the offered
rails to those supporting it; when omitted, no currency filter is applied (setup collects a
payment method, not a charge).

Example: "GBP"

The URL to return to when the customer successfully completed the checkout. If empty, a Synapto hosted
success page is displayed.

When set, a "Back" link becomes visible on the checkout page and the user is redirected to this link. It is recommended to set this field.

Name of a customer to attach to the intent. Required in SETUP mode (the saved payment method is attached
to this customer and can't be used later with any other customer).

Example: "accounts/acct_RwDJsJd8z0exhtfX2570Q/customers/cus_qwjkdnqkwjdnq".