Implement a cXML PunchOut session from PunchOutSetupRequest to OrderMessage for a B2B supplier storefront

Steps

1. Receive a PunchOutSetupRequest HTTP POST from the procurement system (Ariba, Coupa, etc.) — parse the XML body to extract BuyerCookie, the Extrinsic identity fields, and the BrowserFormPost URL.
2. Authenticate the buyer using the shared secret in the <Credential> element; look up the buyer's account and contract pricing tier from your internal database.
3. Return a PunchOutSetupResponse containing a single <StartPage><URL> that encodes a one-time session token — the procurement system will redirect the buyer's browser to this URL.
4. When the buyer clicks Check Out in your storefront, build a cXML OrderRequest (or PunchOutOrderMessage if your store does not process the order natively) and HTTP POST it to the BrowserFormPost URL captured in step 1.
5. Validate the response HTTP 200 and a <Response><Status code="200"> element; log the PunchOut session ID for reconciliation when the buyer later submits the purchase order.
6. Handle the subsequent EDI/cXML 850 Purchase Order that the procurement system sends after internal approval, and acknowledge it with an 855 or cXML OrderConfirmation.