Paypal PDT integration in .Net with Blazor - paypal

Question

We are trying to integrate Paypal subscriptions with .Net web application done in Blazor.
We have created a button that allows the customer to enrol to a subscription defined in paypal business.
The customer can do the "payment" or "enrolment" successfully and it returns back to our web application with the specific URL that we defined.
This return URL contains just one parameter, called "token".
The problem is that we want to be syncronize paypal payments with our own control of payments implemented by us in the application, so we need to receive some other information in this URL like the transactionID or some identifier that we can use later to link with one customer.
We have configured paypal to support PDT.
When the customer finishes the enrolment to the subscription, the token returned to the URL i passed to our backend and with this, we try to get the some extra information.
We do a post to the url "https://www.sandbox.paypal.com/cgi-bin/webscr" passing as parameter the token that we received back from the transaction and our token as merchant in paypal.
Theorically, when doing this call, we should receive a response that contains "SUCCESS" and then, we should be able to extract some parameters configured in the PDT of paypal.
But, unfortunately, we always receive "FAIL"
Do you have any idea why does it happens?
Thank you very much in advance.

Answer 1

PDT returns are not reliable, as a payment can complete and the client return never happen for any number of reasons -- so PDT should never be used for anything important, and thus there is no reason for a well designed integration to bother attempting to verify them. Contents of a PDT is suitable for information purposes (display summary of success) to a customer only -- if you are doing anything else with PDT data, rethink your integration..
If you must continue such a very old checkout method that uses PDT as part of its process, a service such as IPN or Webhooks should be added on to receive a somewhat more reliable, direct post from PayPal with the payment information.
The best solution however, is to do neither of those things -- since adding on asynchronous notifications to an existing very old integration has its own disadvantages. Instead, discard your current integration completely as it's already about 4 generations old / 20 years old, and implement a new, current PayPal Checkout integration from scratch:
Follow the PayPal Checkout integration guide and make 2 routes (url paths) on your server, one for 'Create Order' and one for 'Capture Order'. Both of these routes should return only JSON data (no HTML or text). Inside the 2nd route, when the capture API is successful you should verify the amount was correct and store its resulting payment details in your database (particularly purchase_units[0].payments.captures[0].id, which is the PayPal transaction ID) and perform any necessary business logic (such as sending confirmation emails or reserving product) immediately before forwarding your return JSON to the frontend caller. In the event of an error forward the JSON details of it as well, since the frontend must handle such cases.
Pair those 2 routes with this frontend approval flow: https://developer.paypal.com/demo/checkout/#/pattern/server . (If you need to send any additional data from the client to the server, such as an items array or selected options, add a body parameter to the fetch with a value that is a JSON string or object)