Apply a voucher to a Basket

Redeems a VoucherCode against the basket as a payment method. Trybe-issued vouchers carry a remaining balance and are partially or fully applied to the basket total; external vouchers (from a third-party voucher integration) are validated with the provider and held against the basket until submit.

Returns 400 Bad Request if the voucher can't be redeemed (e.g. wrong site, expired, already fully spent, or the third-party provider returned an error).