Add a woocommerce 'add to cart' popup to authorize third-party product

We are looking to develop a woocommerce pop-up add-on that will verify soft products with third-party vendors. The product that is being sold is a remittance of some kind. For example, if Jack wants to cover $50 of Jill's grocery bill (or restaurant bill...etc), then Jack logs log into our website, makes the $50 purchase, and identify Jill's Cell number as the destination of the PIN that Jill would use to make the payment. If all goes according to plan (i.e. Jill number is verified, PIN can be issued), then the $50 purchase is added to the cart and Jack pays the amount normally from his credit card / paypal account with us at the checkout point. If Jill's number is not verified, then a legal disclaimer shows up to tell Jack that he can proceed at his own risk. Therefore, this popup will perform the following: 1) Make an API call to the 3rd party fulfillment server to validate Jill's cell number via API. 2) Make sure that the supermarket / restaurant in question is actively issuing PINs via API call. 3) Authorize payment from credit card (but not commit. Our purpose here is to insure that Jack can pay the full amount once the PIN is issued). At checkout point: 4) Process payment and issue the PIN via API call. Both items in (4) should happen at the same moment. Using a combination of login/paypal info, Jack is to pay immediately (using (4) above) upon clicking on the 'place order' button. Plenty of wordpress popup plugins (free and paid) are available. the API documentation will be provided. We need help in inserting the code in the right place.