This endpoint processes a signed cart, which has previously been
generated via the /cart/calculate endpoint. For external payments, we will
provide the payment parameters for you to pass in, so that we bypass our internal
payment verification steps.


It is also important to note that if an email confirmation is required, the field email must be non empty in the signed_cart and
an extra field named "send_email_receipt", must be manually added and set to true at the root level of the payload's body.