PayPal Website Payments Standard API

I assume the answer to this is in the PayPal documentation somewhere, but I've been reading it for two days and my eyes are beginning to whirl.

For Website Payments Standard, I create a page on my site (let's call this page A) which contains a dynamically generated form which contains a bunch of hidden fields detailing the content of the cart, prices of each item included, etc. It also has a token to identify the seller id. And one visible form element: the submit image.

The customer clicks the image, which submits the form to PayPal. PayPal do their thing (which includes posting s开发者_StackOverflow中文版ome data back to us at a URL we define (let's call this page B)), and then returns the customer back to us (again at a URL we define (let's call this page C)).

What I'm looking for is some sort of transaction ID. We have hits from the user on pages A and C, and a hit from PayPal on page B. How do we know these three hits apply to the same transaction? (The hit on page C may not happen, of course.)

In a separate question, am I the only one who thinks PayPal documentation is confusingly written?

We can already identify A and C as the same user by means of the session ID. And, on A, we can set a form field called custom containing any information we like. This will be posted back, unchanged, by PayPal to page B. This can be used to identify the user there.