post https://europe-west3-kotanipay-test.cloudfunctions.net/api_v2/transactions/paybill
This endpoint is used by a partner who desires to pay a business
or a merchant for goods sold via paybill.
This endpoint is currently only used in Kenya Kindly note that all
transactions done through paybill number are subjected to incur a transactional
fee that varies per the amount transacted.