API Integration in Drupal Shopping Cart -- 2

We need to integrate an API of vouchers in shopping cart developed in DRUPAL. Requirement: In the shopping cart, when a buyer will select a product/ add in cart or on check out, we will request the Buyer to provide the Voucher number, when he will enter the voucher number, the API will call for details of the voucher, a window will pop up in which Voucher Number and Voucher Amount will be reflected, now the buyer can insert the amount of money which he wants to be charged on the Voucher. Suppose buyer is buying a shirt in USD 100, he has a voucher amounting USD 60, he enters the voucher number and on authetication from the API, he can enter the amount of USD 60, the web portal should then charge the buyer USD 40 on credit card, the voucher usage history will be sent back to the main voucher database through the API, to make the scenario more clear, I am attaching the picture: [login to view URL] [login to view URL] Also, the API Details are also attached. Server Credentials for API [login to view URL] using this Uri different websites can use vouchers for payments of products by adding service reference of this uri. Server Credentials for Client Application [login to view URL] /**********************************************************************/ Vouchers List for testing: 1) TAg4NXBazJI1dZGXjZRP 2) b9GQIyLG37yLXlupGG2K 3) gNMETC82SxJ0Kmb3I8Cf 4) or47saiu4S1wyIgI441O 5) HvZ5iyiGP98I138DElTK One has to simply add the Service Reference with the Website and then can consume the API Methods. Please check these below Reference Articles that may help. [login to view URL] [login to view URL] Once its done, we also expect the developer to tell us in bullet points how exactly he has incorporated the API.