@type t() :: %PolarExpress.Schemas.CheckoutBillingAddressFields{
  city: PolarExpress.Schemas.BillingAddressFieldMode.t() | nil,
  country: PolarExpress.Schemas.BillingAddressFieldMode.t() | nil,
  line1: PolarExpress.Schemas.BillingAddressFieldMode.t() | nil,
  line2: PolarExpress.Schemas.BillingAddressFieldMode.t() | nil,
  postal_code: PolarExpress.Schemas.BillingAddressFieldMode.t() | nil,
  state: PolarExpress.Schemas.BillingAddressFieldMode.t() | nil
}