I'm having some trouble choosing between PayPal's Instant Payment Notification (IPN) and Payment Data Transfer (PDT).
Basically, users buy a one-off product on my site, pay on PayPal, and return to my site. I understand how IPN works but I'm now seeing that I might be able to trigger the various actions that take place after a successful purchase more easily with PDT, as the data gets returned there and then (as opposed to needing a separate listener).
However, PayPal's PDT documentation contains this cryptic line: "PDT is not meant to be used with credit card or Express Checkout transactions." ... but I can't find anything further whatsoever on the topic.
Are credit cards REALLY not meant to be used with PDT? I would like more than a sentence.
Does that mean that a user must have/create a PayPal account to pay?
Does it mean that if I want to allow users to pay with their PayPal accounts AND/OR with credit cards directly, I must implement IPN?
Could anyone who's gone through this kindly shed some light?
The APIs for PDT and IPN are similar. The main difference is when you receive the notification. For that reason I would recommend implementing both.
Implement both and get the best of both worlds. But if you're only doing one, IPN is the reliable one.
One catch: if you implement both then there's a chance your payments could be processed twice. Take care to ensure that doesn't happen. The application I wrote handles the PDT and IPN almost identically (the backend part is the same) and that code acquires a per-web-user lock in the database, so that if the same user tries to submit the exact same payment multiple times it can only be processed once. Once processed the result of that process is re-used for any subsequent attempts to process it.
Edit One more thing: IPN carries more information than PDT. There are lots of different messages that you can receive from IPN, such as chargeback notification, etc, and thus you really should implement it.
PayPal's PDT system sends order confirmations to merchant sites that use PayPal Payments Standard and lets them authenticate this information. Such sites can then display this data locally in an "order confirmation" page.
IPN provides the same capabilities described above. So, when should you choose PDT instead of IPN?
With PDT, your site is notified immediately when a customer completes payment. With IPN, however, there is a material lag between the time a customer completes payment and the time your site receives notification of this event.
So, use PDT if your site includes a feature that requires immediate payment notification.
For example, consider a digital music store. With PDT, this store can let customers download their purchases right away since PDT sends order confirmations immediately. With IPN, such immediate order fulfillment is not possible.
PDT has a a major weakness: it sends order confirmations once and only once. As a result, when PDT sends a confirmation, your site must be running; otherwise, it will never receive the message.
With IPN, in contrast, delivery of order confirmations is virtually guaranteed since IPN resends a confirmation until your site acknowledges receipt. For this reason, PayPal recommends that you implement IPN rather than PDT.
Another advantage of IPN is that it sends many types of notifications, while PDT sends just order confirmations. So, using IPN, your site can receive, for example, chargeback notifications as well as order confirmations. Note: If your site must be notified of payments immediately, you can implement both IPN and PDT. However, if you do, your site will receive two order confirmations for each sale. As a result, you must be careful to take action (say, ship a product) on just one copy of a given confirmation message.